First, Barclay's Bank runs the MasterCard program so I'm assuming you were talking to Barclay's Bank not US Airways people?

Second, credit card merchants absolutely can require a minimum for using a credit card and most do. They can not require a minimum for using a debit card. (I stand corrected on that - they can't require minimums even though it's a common practice.)

Third, restaurants, even in the US can add gratuities to your bill. Some in New York now add 18-20% on parties of 5 or more.

Finally, there often is a foreign exchange surcharge added by the credit card company - not 5% but still a surcharge for the foreign exchange transaction.