So we had a merchant do this while traveling in Costa Rica. They gave us receipt that showed the surcharge. When we got home I faxed it to MC and got the surcharge credited back. The merchant got spanked.

Many are getting around this by listing everything as cash prices then wacking you.

The bottomline is that under the terms of agreement merchants can only offer a cash discount - they have to charge the same amount to all. If you want to see some nastiness try this while selling ebay.