When purchasing items in Europe, you will be charged a Value Added Tax, which can be significant (up to 25%).  Foreign visitors can usually get reimbursed for this cost in several ways but you must request a form from the merchant where you purchased the item(s).

You will then need to complete it with your name, address and passport number and present it along with your receipt(s) to a Customs office at an international airport, port or border before departing Europe for the U.S.  If the country is not in the European Union, you must get it stamped when leaving that country before traveling to the next.  Otherwise, purchases made in the EU countries can be accumulated and stamped by one Customs officer in your country of departure.  They may ask to see the unused item(s) so make sure you don’t put it in check luggage first.  The Customs officer will stamp the form, which you then must take to a tax refund office at the airport, port or border OR mail it in the envelope provided.

If the amount of your purchase is large enough, this is worth taking the time – but be sure to allow some extra time at the airport.