If an order is prepaid and only some items have been marked as despatched, you can only search for these invoice lines in the bottom option on Sales Invoices>Avail Lines (will show as type E for Prepaid)
This is because there is no facility to do multiple payments with Paya with one prepay authorization.
Therefore if the order is not fully delivered and you accidentally invoice the delivered lines – it ‘won’t’ let you use that card payment to invoice the backordered items later.