Another possibility is to use the Sample Gateway. It collects the credit card information but doesn't actually do any authorizing; it just returns 'success' for everything.
This would lead to all the orders in the eCommerce system looking like the card has been charged when it has not been, but if your client is going to process the cards offline then they are going to have to have some procedure to cross-reference the cleared payments with orders anyway. Orders which prove to have had invalid credit cards can be voided.
