How do you record payments on your invoices? In a field in the invoices table or a related payments table?
How do you identify each customer so that you can find all outstanding invoices for a given customer? By Name or by CustomerID? (Names aren't unique so you don't want to deny service to one customer simply because a customer of the same name hasn't paid their bill.)
Yes, in a field in a related payments table and each customer is identified by a customer ID
I'm guessing that you have this already defined in Invoices:
cTotalpayments: Sum ( PaymentTable::PaymentAmount ) //computes total payments for current invoice
cAmountDue: InvoiceTotal - cTotalpayments //computes amount due on current invoice
IF so, this calculation defined in your customer table would compute the total owed for any given customer:
Sum ( Invoices::cAmountDue )
Comparing that to a credit limit field in the same table would tell you who has gone over the limit and who has not.