However, some countries impose duty on goods brought into the country, though they had been bought duty-free in another country, or when the value or quantity of such goods exceed an allowed limit. Duty-free shops are often found in the international zone of international airports, sea ports, and train stations but goods can also be bought duty-free on board airplanes and passenger ships. They are not as commonly available for road or train travelers, although several border crossings between the United States and both Canada and Mexico have duty-free shops for car travelers. In some countries, any shop can participate in a reimbursement system, such as "Global Blue" and "Premier Tax Free", wherein a sum equivalent to the tax is paid, but then the goods are presented to customs and the sum reimbursed on exit.