VM upgrade of Payment

1. one product X, need to have the option of payment; Billed (recive a bill in the package). This should be possible to choose if a user select ONLY that product. 2. if the customer choose the payment PAYMENT-IN-PO (payment as they pick it up at postoffice), the shipping need to be automatically chosen to PAYMENT-IN-PO (same name and function at the post-office) i.e.: if they choose the payment PAYMENT-IN-PO, they Cannot Choose shipment, but there is one selected..