How to Migrate Your Invoices From a Niche App to an Excel Invoice System (With Reminders)

Stop losing time to export/import chaos — migrate invoices from that niche app into a single, auditable Excel system with automated payment reminders

If you run a small UK business you already know the pain: a niche invoicing app holds years of invoice history, another system tracks payments, and reminders live in a third place. Consolidating that data sounds risky and technical — but with the right plan, a ready-made template, Power Query and a simple macro you can migrate, standardise and automate reminders in a single Excel workbook in a day. If you feel overwhelmed by tool bloat, this approach will help you rationalise.

What this guide gives you (fast)

• Step-by-step export strategy for CSV, JSON or API-based invoice histories.
• Power Query tutorial to import, normalise and combine exports — including an M code snippet.
• Macro-based reminder system that sends Outlook emails, logs reminders and updates invoice status.
• Practical UK-focused compliance notes (VAT records, 6-year retention) and governance best practice.
• A downloadable template bundle for finance, invoicing and reminders tailored to UK small businesses (link below).

Why migrate invoices to Excel now — 2026 trends you should use

In late 2025 and into 2026 several trends make an Excel-centric approach compelling for small businesses:

• Power Query and Excel connectors have matured: they now handle larger CSV/JSON loads and more API auth flows, making import automation easier.
• Rising subscription costs and tool bloat mean many businesses are consolidating to fewer, more controllable systems.
• More UK small businesses are prioritising auditability and local control of VAT and accounting records as HMRC digital compliance expectations continue to emphasise reliable records.
• Power Platform and low-code automation (Power Automate) are available, but a lightweight Excel + VBA approach is still fastest to implement and keeps data on-premise or in your chosen cloud storage. For approaches to cloud and pipeline readiness that scale, see this case study on cloud pipelines.

Quick checklist before you start (do this first)

1. Pick a test set: export 30–100 recent invoices to validate mapping and reminders.
2. Confirm retention: UK VAT records must be kept for at least 6 years.
3. Decide where the master workbook will live (OneDrive/SharePoint for co-authoring, or local network for controlled access).
4. Backup the niche app data export and store a copy securely (GDPR awareness: personal data protections). For ethical handling of scraped or exported data see guidance on ethical data practices.

Step 1 — Audit the invoice data model in your app

Before exporting, document the fields you need. Common invoice fields:

• Invoice ID, Invoice Date, Due Date
• Customer ID, Customer Name, Customer Email
• Line items (description, qty, unit price), Subtotal, VAT, Total
• Payment status, Payment date, Payment method, Transaction ID
• Notes, Tags, Currency

Create a mapping sheet in a temporary workbook that lists source field names and the target Excel field names. This will be your translation table when loading in Power Query.

Step 2 — Export invoice history (CSV, JSON or API)

Most niche apps let you export CSV. If you only have API access, you can pull data via the app’s API and save JSON/CSV. Use the simplest export that includes each invoice and payments.

CSV export (recommended)

1. Export complete invoice rows (one row per invoice). If your app requires separate line-item exports, export both invoices and line_items.
2. Include payment records if possible; otherwise export payments into a separate file.
3. Name files with dates, e.g., invoices-2024-01-01_to_2025-12-31.csv.

JSON / API export

If you must use the API, fetch pages and write to JSON/CSV. Example curl for a paged API (replace tokens):

curl -H "Authorization: Bearer YOUR_TOKEN" "https://api.nicheapp.com/invoices?page=1" -o invoices-page1.json

Combine JSON pages into one file or import each page into Power Query and append.

Step 3 — Prepare the Excel template (structure you’ll use)

Download our template bundle for UK small businesses (invoice migration template, invoices table, payments table, reminders sheet, ageing dashboard). Your workbook should contain these sheets:

• _Config — mapping table, VAT codes, currency format, date format settings.
• Invoices_RAW — Power Query landing table (do not edit manually).
• Payments_RAW — if payments exported separately.
• Invoices — cleaned, canonical table driven by queries.
• Reminders — template for emails, schedule and log of sent reminders.
• Dashboard — ageing, outstanding totals, KPIs.
• Lookup — customers and mapping helpers.

A good template enforces consistent column names and types so formulas, pivots and macros won’t break.

Step 4 — Use Power Query to import and normalise

Power Query is ideal for import because you can repeat transformations and refresh after new exports. Key goals:

• Combine multiple CSV/JSON files into one query
• Standardise date formats and currency values
• Map source field names to your canonical names
• Remove duplicates and validate totals

Power Query steps (UI + M snippet)

1. Data > Get Data > From File > From Folder (if you export monthly files to a folder).
2. Combine Files > Transform Sample File to standardise columns.
3. Use Merge or Append to join invoices and payments if exported separately.
4. Use the mapping table from the _Config sheet to rename columns with a simple Join/Expand pattern.

Here’s a compact M snippet to combine CSV files in a folder and clean common fields (paste into Advanced Editor):

let
    Source = Folder.Files("C:\\exports\\invoices"),
    CsvFiles = Table.SelectRows(Source, each Text.EndsWith([Name], ".csv")),
    GetBinary = Table.AddColumn(CsvFiles, "Content", each Csv.Document(File.Contents([Folder Path] & [Name]),[Delimiter=",", Encoding=1252, QuoteStyle=QuoteStyle.Csv])),
    Expand = Table.ExpandTableColumn(GetBinary, "Content", {"Column1","Column2","Column3"}, {"col1","col2","col3"}),
    // example rename - map real field names below
    Renamed = Table.RenameColumns(Expand, {{"col1","InvoiceID"},{"col2","InvoiceDate"},{"col3","TotalAmount"}}),
    ChangedTypes = Table.TransformColumnTypes(Renamed,{{"InvoiceDate", type date},{"TotalAmount", type number}}),
    RemoveDupes = Table.Distinct(ChangedTypes, {"InvoiceID"})
  in
    RemoveDupes

Adapt column names and types to your source. Use UI transforms to parse dates using Locale = English (United Kingdom) if needed.

Step 5 — Reconcile payments and create an ageing view

If payments were exported separately, merge them in Power Query by InvoiceID. Create calculated columns:

• BalanceDue = Total - SUM(Payments)
• DaysPastDue = Today() - DueDate
• Status = Paid / Part-Paid / Overdue / Upcoming

Use a Pivot Table or the Dashboard sheet to show:

• Outstanding by ageing bands (0–30, 31–60, 61–90, 90+)
• Top 10 debtors
• Monthly invoices and cash flow from payments

Step 6 — Add payment reminder automation via macros

We provide a compact VBA macro that:

1. Scans the Invoices table for overdue balances
2. Generates personalised email bodies using the Reminders sheet templates
3. Sends emails via Outlook, logs the sent reminder and updates a ReminderSentDate field

Insert this VBA in a standard module (Alt+F11 > Insert > Module). This sample assumes your Invoices table is an Excel Table named "Invoices" and you have Outlook installed:

Sub SendOverdueReminders()
  Dim ws As Worksheet
  Dim tbl As ListObject
  Dim oOutlook As Object
  Dim oMail As Object
  Dim r As ListRow
  Dim dueDate As Date
  Dim daysPast As Long

  Set ws = ThisWorkbook.Worksheets("Invoices")
  Set tbl = ws.ListObjects("Invoices")

  On Error Resume Next
  Set oOutlook = GetObject(, "Outlook.Application")
  If oOutlook Is Nothing Then Set oOutlook = CreateObject("Outlook.Application")
  On Error GoTo 0

  For Each r In tbl.ListRows
    If IsNumeric(r.Range.Columns(tbl.ListColumns("BalanceDue").Index).Value) Then
      If r.Range.Columns(tbl.ListColumns("BalanceDue").Index).Value > 0 Then
        daysPast = Date - r.Range.Columns(tbl.ListColumns("DueDate").Index).Value
        If daysPast > 7 Then ' change threshold as required
          Set oMail = oOutlook.CreateItem(0)
          With oMail
            .To = r.Range.Columns(tbl.ListColumns("CustomerEmail").Index).Value
            .Subject = "Payment reminder: Invoice " & r.Range.Columns(tbl.ListColumns("InvoiceID").Index).Value
            .Body = "Dear " & r.Range.Columns(tbl.ListColumns("CustomerName").Index).Value & vbCrLf & vbCrLf & _
                    "Our records show invoice " & r.Range.Columns(tbl.ListColumns("InvoiceID").Index).Value & " is overdue by " & daysPast & " days." & vbCrLf & _
                    "Amount outstanding: " & Format(r.Range.Columns(tbl.ListColumns("BalanceDue").Index).Value, "£#,##0.00") & vbCrLf & vbCrLf & _
                    "Please make payment or contact us to discuss." & vbCrLf & vbCrLf & "Kind regards," & vbCrLf & "[Your Business]"
            .Send
          End With

          ' Log reminder
          r.Range.Columns(tbl.ListColumns("ReminderSentDate").Index).Value = Date
        End If
      End If
    End If
  Next r

  Set oMail = Nothing
  Set oOutlook = Nothing
  MsgBox "Reminders processed.", vbInformation
End Sub

Key notes:

• Test using a non-production mailbox or change .Send to .Display while testing.
• Adjust the daysPast threshold for first, second and final reminder logic.
• Add logging to a Reminders sheet with columns: InvoiceID, EmailDate, TemplateUsed, Outcome.

Advanced: Multi-step reminder flows

Create multiple reminder templates and store them on the Reminders sheet with a ReminderLevel column. Extend the macro to pick the correct template based on DaysPastDue and ReminderCount.

Step 7 — Scheduling and safe automation

Options to automate reminders:

• Use Application.OnTime inside Excel for a basic scheduled run (workbook must be open).
• Use Windows Task Scheduler to open the macro-enabled workbook on a schedule and run an Auto_Open or Workbook_Open macro to call the reminder routine.
• For cloud users, consider Power Automate. In 2026 it's common to pair Excel tables on SharePoint with Power Automate flows for robust scheduled emails without local machines — and prepare SaaS and community platforms for scheduled tasks and outage handling as part of your runbook: Preparing SaaS for outages & mass-user confusion.

Example Workbook_Open trigger:

Private Sub Workbook_Open()
    Call SendOverdueReminders
  End Sub

Step 8 — Test, validate and go-live

1. Run the process on your test set and verify totals match the app’s reports.
2. Confirm currency and VAT calculations.
3. Send test reminders to a controlled inbox and confirm formatting and attachments.
4. Create a rollback plan to keep the niche app as canonical until you confirm the Excel dataset is complete and accurate.

Pro tip: Migrate in phases — start with the last 12 months of invoices; once reconciled, import older batches in chronological order. This lowers risk and keeps recent reporting intact.

Governance, security and UK compliance

When migrating invoice data, protect customer information and comply with UK rules:

• Retention: Keep VAT records for at least six years (HMRC expectation).
• GDPR: Limit access to the workbook, store on secured OneDrive/SharePoint with MFA where possible. For audit trail guidance that applies to sensitive intake workflows, see audit trail best practices.
• Backups: Keep regular, versioned backups. If you use SharePoint, enable version history — or evaluate cloud NAS and object storage options to meet your retention and restore objectives: Cloud NAS field review and object storage provider review.
• Audit trail: Use a Reminder log and track who changed which data. Consider protecting key sheets and cells with a password.

Troubleshooting common problems

• Dates import as numbers or wrong years — set the Locale in Power Query and use Date.FromText with the correct format.
• Duplicate invoice IDs — check for different file encodings or trailing spaces; use Trim and Clean transformations.
• Outlook refuses to send programmatically — check Trust Center settings and use .Display first for testing. If you're building resilient automation, consider how outages and permission changes affect scheduled email runs: see SaaS outage readiness.
• Large datasets slow — filter during import (e.g., last 5 years), or load to the Data Model and use Power Pivot for dashboards. For scaling storage and performance considerations, review object storage options: Top object storage providers.

Real-world mini case study

Jane runs a UK web design studio. She used a niche app for invoices and a bank export for payments. Exports were monthly CSVs. After a one-day migration she consolidated 3 years of invoices into Excel, created a simple ageing dashboard and ran the reminder macro. Result: 30% reduction in late payments in three months and lower subscription costs from pruning two redundant apps.

Actionable takeaways — 10-minute checklist

1. Export a 30–100 invoice test file now and store it in a folder called \exports\invoices.
2. Download the Excel invoice migration template bundle from our site.
3. Import the CSVs into Power Query using the folder method and validate column mapping.
4. Reconcile one month of payments and confirm balance calculations. Consider payment-data compliance checklists for sensitive workflows: payments compliance checklist.
5. Run the reminder macro in test mode (.Display) and adjust templates.
6. Schedule the macro via Task Scheduler or move to Power Automate for cloud scheduling.
7. Enable backups and set access controls (MFA recommended). For file management patterns and versioning approaches, see file management guidance.

Where to get the template & next steps

Our downloadable Invoice Migration Bundle for UK Small Businesses includes:

• Migration template workbook (Power Query ready)
• Reminders automation macros and sample templates
• Quick-start guide and mapping checklist

Download the bundle, run the 30–100 invoice test and book a 30-minute template setup call if you want us to walk through your exports and mapping. If you plan to pitch this improvement internally or to a client after migration, our guide to pitching to larger teams may help structure your case.

Final thoughts — modernise without overcomplicating

2026 is a consolidation year for many small businesses. You don’t need to rip out every tool — you need a reliable, auditable master for your invoices and a simple automation layer that reduces manual chasing. Power Query gives the repeatable import path; a lightweight VBA reminder macro gives immediate automation without extra subscriptions. Migrate in phases, protect data, and keep the niche app as a fallback until you’re confident.

Ready to migrate? Download the Invoice Migration Bundle and follow the step-by-step template. If you want hands-on help, book a migration review with our Excel specialists at excels.uk — we’ll map, migrate and automate your reminders so your cashflow improves and your tools shrink.

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