My merchent charges a percentage for credit cards and a set fee for debit cards. So for me if its a large transaction its better they pay by debit card, and if its a small one id rather pay a percentage. But then i dont get the choice, its upto the customer how they want to pay.
same here, it must be the way the all merchant account providers do it. Like someone above said, you don't get the choice of how they pay.
In the shop I work in though, some people ask which type of card I prefer (its rare though, and often on high sales that they ask.). If its something with a high price I will of course say debit card, as the 60p for a switch card on 1 £100 sale is better for us than a 2% charge on a visa card.