Help with Invoice/ Payments in a DB

[Dav43]

I have a working database that can track inventory, do invoices and the like, but now I want to apply payments to the invoices. Up til now, I have been reentering condensed invoices into Quickbooks, and applying payments there. Most customers pay at the end of the month, so one cheque covers several invoices. The sample MS Order Entry DB is not the way I want to go. I would prefer to have a form that works the same as Quickbooks handles payments. I have a query that totals the open invoices per customer. Now I want to have a form that shows the cheque detail, and then have a subform that shows the current open value of each invoice, and be able to appy a payment (or more importantly a part payment) directly to that invoice. I have tried allsorts of methods, and have a crude form that sort of does what I want with three subforms (one for the open invoices, one for the paymentID details tying all the invoices to the payment, and one calculating all the part payments previously applied to each invoice.), but I need it to be much more simple.

I am using four tables in this process, Orders, OrderDetails, Payments and PaymentDetails.

Does anyone have any ideas? I am sure that this is a common procedure in many applications.

Thankyou for any help in advance.

David