I believe - although I could be wrong on this, so correct me if anyone knows - that the contract/agreements between the shop and the card processing companies forbid the shop owner from charging an extra fee to use the card.
But at the same time, I can see why little shops do it - the card use charge can be more than the profit.
But at the same time, I can see why little shops do it - the card use charge can be more than the profit.