[GNC] Reconciliation Report -> Trade Debtors -> pairing transactions

[flywire]

This is similar to rental payments but my experience is with cash
accounting. It is easy to skip an invoice and in my jurisdiction, this
eventually means it isn't required to be paid. I'd suggest providing a
statement instead of pairing payments and invoices so payments are applied
to the oldest invoice.

An automated system will be better than manual manipulation of a
spreadsheet because errors can be expensive.