The solution depends a bit on whether it is 70K customers or just a few customers with 70K payments.There is an Apply All option in Apply Payments and Memos that will automatically apply a payment to the oldest documents sequentially. There are a couple of methods to import the application data but it would require some manipulation and you would need either SQL programming experience or the Data Porter or maybe Data Import modules. Since most of that Apply All logic is contained in the UI code, a decent developer could build a mod to scroll through the data set and automatically apply it (it's an easy Customizer mod).

Thinking out of the box in practical terms though, it does appear you have a scenario where the customers should have been configured with a Balance Forward billing type. If nothing else, this is something to research and test if you don't want to go down the custom development path. If it is the 70K customer scenario you would still need a way to change some customer attributes in bulk through the back-end. Or, if they are just a large number of POS customers, there's always the possibility of merging the bulk of them into a single account then Apply All would work really well.

Okay, there might be some people rolling their eyes at this point, so although the problem is not insurmountable, you just need to have some extra skills or do some additional testing to come up with the right solution.

It is 70K different customers, no payments have been applied since 2018

It is 70K different customers, no payments have been applied since 2018

Oh, that's classic! You need to make some decisions on how you want to proceed. The only way to effectively handled that much data is with either back-end manipulation or front-end modification or a bit of both. Cleaning up the current data set is a concern but you also need to contemplate what you will want to do if this is a continuing problem, and how you want to approach the purge of any of that data.

I didn't spend much time with this so no error trapping, it's not pretty, etc., but you could add a button to the toolbar in Apply Payments and Memos using Customizer, then add this code to the button and see if it works for you. I don't have much data to try it.

    Dim rs
    Dim sSQL
    Dim dataSetSize
    Dim sCompanyID
    Dim moStrings

    dataSetSize = InputBox("Enter Number of Pmts", "AutoApply Pmts", 0)

    If dataSetSize = 0 Then
        Exit Sub
    End If

    With Form.moForm.oClass
        Set moStrings = CreateObject("StringUtils.CStringUtils")
        sCompanyID = moStrings.gsQuoted(.moSysSession.CompanyId)
        sSQL = "SELECT DISTINCT " & _
                " TOP " & dataSetSize & _
                " [tarCustomer].[CustKey], [tarCustomer].[CustID], " & _
                " [tarCustPmt].[CustPmtKey], [tarCustPmt].[TranID] " & _
                " FROM [dbo].[tarCustomer] " & _
                " INNER JOIN [dbo].[tarInvoice] " & _
                " ON [tarCustomer].[CustKey] = [tarInvoice].[CustKey] " & _
                " INNER JOIN [dbo].[tarCustPmt] " & _
                " ON [tarCustomer].[CustKey] = [tarCustPmt].[CustKey] " & _
                " LEFT OUTER JOIN [dbo].[tarPendCustPmtAppl] " & _
                " ON [tarCustPmt].[CustPmtKey] = [tarPendCustPmtAppl].[ApplyFromPmtKey] " & _
                " WHERE NOT [tarInvoice].[Balance] = 0 " & _
                " AND NOT [tarCustPmt].[UnappliedAmt] = 0 " & _
                " AND [tarPendCustPmtAppl].[ApplyFromPmtKey] IS NULL " & _
                " AND [tarCustomer].[CompanyID] = " & sCompanyID & _
                " ORDER BY [tarCustPmt].[CustPmtKey] ASC; "

        Set rs = .moAppDB.OpenRecordset(sSQL, 1, 0)
        rs.MoveFirst

        While Not rs.IsEOF
            Form.Controls("cboApplyFrom").ListIndex = 0
            Form.Controls("txtCustID").Text = rs.Field("CustID")
            Form.Controls("TxtCustKey").Text = rs.Field("CustKey")
            Form.Controls("txtAppDocNum").Text = rs.Field("TranID")

            Form.Controls("chkShowInvoices").SetFocus
            If Form.Controls("chkShowInvoices").Value = 0 Then
                Form.Controls("chkShowInvoices").Value = 1
            End If

            CreateObject("WScript.Shell").PopUp "Calculating Applications", 1, "Delay for Steve", 4096
            If Not Form.Controls("CurUnappliedBal") = 0 Then
                Form.moForm.moPmtApp.Process_AutoApplyClick
            End If

            Form.moForm.HandleToolbarClick "K8"
            rs.MoveNext
        WEnd
    End With

    If Not rs Is Nothing Then
        rs.Close
        Set rs = Nothing
    End If

    MsgBox "Run Complete"