Are there any apps available for amending "existing" customer records using the API or is it definitely not possible?
Hello there, dwhite2.
As far as what we're doing in QuickBooks Online, we'll need to enter the transactions (such as credit memo, refunds, invoices, and etc.) manually if we want the customer balances to change. You might find these articles helpful:
Meanwhile, you can click the Apps menu and search for possible applications that can help you achieve your goal. Aside from that, we also have the Intuit Developer page. There are QuickBooks users, engineers, and developers that can also share their ideas about the third-party application you want to use.