Changing " Payment Request" to "Invoice for progress payment"

I have had many corporate clients giving me problems with the wording as it's insufficient for submitting a payment. They require it to show as an invoice.

I think this can be simply resolved with a change of wording so to not cause confusion with these contracts.

I am sure I am not the first one experiencing this problem, and I'll definitely not be the last. But someone has to say it.