If you pay by credit card and you find there is any kind of problem with the retailer, you can lodge what they call a 'chargeback' against the retailer with your credit card company. If the retailer fails to sort the problem out or issues a credit of their own free will, then the credit card company can take the money back from the retailer and credit the money to your account. I always find it amusing how quickly problems can be resolved when you threaten to make a chargeback against a retailer. I guess if the retailer gets too many chargebacks, then their merchant account would be suspended so they could no longer accept credit cards.
Talking about overcharging, a friend of mine was in Andorra last week and stopped for a coffee and got presented with a bill for 842 Euros!!
They apologised and gave him the correct bill!