A surcharge is an extra amount of money you have to pay when you buy something. If you purchase your concert tickets online instead of at the box office, you'll have to pay a five-dollar surcharge.
Any added fee can be called a surcharge, whether it's the surcharge you pay for bringing an extra suitcase on an airplane or the surcharge a business pays every time a customer uses a credit card. As a verb, it means to charge someone an extra fee: "I'm changing banks because mine surcharges me every time I use my card at an ATM." In the 15th century, surcharge meant "overcharge" or "charge too much."