Why would you want the same serial numbers?

OrderEntry calculates the (true) balance outstanding for a customer before it worries about whether it needs to do a credit check.

We will investigate whether the number is expected elsewhere in the program. My guess is that it must work this way because you might change the TERMS to one that does require a complete credit check.

It should only be getting "slower" for this one high volume client. How many orders/invoices are you processing a day for them? Why not just add additional items onto the first order for the day and invoice once?