This may not be new or news to some, but I've never seen it mentioned.
Went to the US yesterday and purchased some stuff at Cycle Barn. If you pay by credit card, Cycle Barn puts their own exchange rate, which is much worse than what your card would give you. The current rate is about $1.06, the Cycle Barn rate is about $1.12. Depending on the size of your purchase thats a huge difference.
They don't tell you, but you have the right to refuse their rate and take the credit cards company's rate. You have to do this before they ring it through and you sign. Apparently they won't know how to do this, be persistent, they will call customer service and do it for you, eventually. This saved me some money.