Below is a copy of my Payments.
Don't see any such copy.
I would link your orders table to a related table of payments by order ID. It's not clear where "invoices" come into that, but maybe that's the name you are using where I am describing a related table of payments due to creating and sending out an invoice for each scheduled payment due.
Using "Invoices" as the name of the related table of payments, you can set up this relationship:
Orders::__pkOrderID = Invoices::_fkOrderID
Then you can define this calculation field (NOT an auto-entered calculation):
Orders::OrderTotal - Sum ( Invoices::Payment )
to compute the remaining balance on any given order.
For an explanation of the notation that I used, see the first post of: Common Forum Relationship and Field Notations Explained