Last time I ordered, I found out when I got my credit card statement that Citibank adds a 3% fee to PBK (and any other foreign) purchases. (The exchange rate that Citi applied was also slightly less favorable than what PBK used, not surprisingly.) The tires still come out cheaper, but Citi certainly takes their "vig." Do other card issuers charge the same fee?

Finally, as has been mentioned before, it's worth using a "virtual account number" if you can when ordering from PBK.