This is nothing to worry about and is actually quite common.
When you buy anything with a credit card the retailer is charged a small percentage, usually 2.5%. New legislation means that the retailer has to declare that this charge is being made and in effect, justify where it has come from. There is absolutely no difference in the actual price you pay, whether in cash or by card, the 2.5% is merely an "overhead" that the retailer has already taken into account when setting the purchase price.
This is one reason why it is always worth haggling over the price in stores (not online) as any retailer that says "We cannot offer a discount" is lying - simply say that for a 2.5% discount you will pay cash - no discount then you will pay by card. The retailer will loose this 2.5% whichever method of payment you then use.