iSupplier Portal: Unable to choose payment term when submits electronic invoice from iSupplier Porta

Hi,

Our most of the payables invoices are coming from iSupplier Portal(source = ISP)  and we are heavily depending on electronic invoice. So, a vendor may have a PO set up for net 30 contract terms which would be used in subsequent invoices.  Vendor  can create and submit invoice agaist the PO but there is no payment term available option to choose different payment term(for faster payment and discount). For an instance,  at the point of invoice entry they could choose a payment term such as .5%/10 Net 30 instead of standard net 30 without the contract changing.  Future invoices would still default to