The real question is why aren't all payments applied to invoices after they have been created for the same customer? As an aside but associated is why are some payments credited to invoices when they shouldn't be as the program sometimes does one, and other times the other, with no known or evident consistency in application. If there were consistency then handling the aberrations would be much simpler. 

But then again, a simple reminder that

A: there is a credit associated with the customer

or B:  there are multiple invoices that a payment could be applied to would go a long way to making the program useful.

Perhaps I expect too much of a program that is supposed to simplify this data processing?

Disappointed again.

Cheers!