At a previous employer we had the same problem... I cheated. Since the customer also was on One Invoice per PackSlip, I captured the packslip number during the invoicing process, and created a fake "Invoice Number" that printed on the AR Invoice Form in the Invoice Number area. Then, in smaller print over on the right-hand side, I added a field called "legal reference" and there I showed the Epicor invoice number. I also created a dashboard so my AR people could quickly refer to either one to get the other.
Ernie Lowell