YouTube videos and Knowledge Base articles exist regarding the new Admin Portal and Billing Portal for Transaction Pro Online.
Search "Admin Tool" to locate them or email email@example.com with any questions.
Is there a way to set Transaction Pro Importer Importer to not process payments if it will result in an overpayment?
No, QuickBooks Desktop (QBDT) will make any overpayment a credit on the Customer's account.
Use Transaction Pro Exporter to export a list of Invoices and compare to the Receive Payment import file. Then filter the import file for just the Receive Payments that do not result in an overpayment.