Hi @margiemalone ,
To make the database clear, separate lists are usually used.
1. Customers list
2. Payments list with columns customer LookUp, paid amount and related invoice LookUp.
3. Invoices list with columns customer LookUp, invoiced amount Number and paid amounts Number, or additional multiple LookUp column of payments to link to payments list.
Whether in PowerApps or Power Automate, with these three list it will be quite easy to manage the project.
Best regards,