From the description of ripple, I believe that an unusual system of payments for merchant's goods is possible, not quite like the ususal one in a fixed currency.
(So here I'll describe this interesting system, leaving the answer concerning organizing a traditional payment system for a customer to other people.)
What if the merchant's goods is the curency? Similar to bonus systems some merchants offer. Then the merchant should actually act as a gateway (shouldn't they?), and redeem real goods for their IOUs. And their IOUs can be traded by other people for other currencies, the price depending on how much the goods from this merchant are wanted, and how much they trust the merchant (that the merchant will keep the promise).
So, the merchant would initially sell their IOUs for other money, or give them out according to their wishes, and then these IOUs can be traded, and returned back to the merchant for the goods.
If the merchant constantly sells these IOUs, it shouldn't be a problem for someone to pay with other money, because he would be able to immediately buy the merchant's IOUs in ripple, and then redee, them for the goods.