When you swipe your card for purchasing something, merchant has to pay between 1.5% to 2.5% service charge depends on their industry and which card customer is holding.
If customer is coming with Amex or Diners/Discover card merchant needs to pay 3.5 % to 4% as service charge.
This is the service charge for credit card machine.
These charges goes out from merchant’s profit.
If it is a small payments like around AED 200 its ok ,they can adjust from the profit, but if transaction amount is AED 20000, what will do?
Charge will be AED 500, so in this case merchant may ask you for the service charge extra.
And for some Industries like Mobile phone trading , their profit margin will be very less may be 1.5% , in this case if customer use card , shop owner needs to pay 2.5% to banks. So he will be in loss. So they will ask for additional service charge.
But anyhow as per the agreement between merchant and Credit Card Swipe Machine company, they are agreed that they will not charge any extra amount from customers for card payment.