A surcharge is an extra cost or fee added to a bill or price. It's like a hidden additional charge that might catch you off guard.
When shopping online or at the store, you might see a "free shipping" deal that sounds great, but then they surprise you with a surcharge for handling or packaging. Sometimes airlines will add a surcharge if you book your ticket last minute or change your flight plans. In these cases, the original price didn't include the extra fee, and it's added later as a surcharge. If someone says that their phone bill has a surcharge for data usage, they mean there's an additional charge for using more than the expected amount of internet on their phone.
How common is "surcharge"?
Word surcharge is considered rare in modern English. It appears most frequently in newspaper texts and less often in other writings.
Definitions
noun
An addition of extra charge on the agreed, stated, or baseline price.
Example: Our airline tickets cost twenty dollars more than we expected because we had to pay a fuel surcharge.
The part of the price of a subsidized good or service that is not covered by the subsidy and so must be paid by the consumer.
An excessive price charged e.g. to an unsuspecting customer.
(philately) An overprint on a stamp that alters (usually raises) the original nominal value of the stamp; used especially in times of hyperinflation.
(art) A painting in lighter enamel over a darker one that serves as the ground.
(law) A charge that has been omitted from an account as payment of a credit to the charged party
(law) A penalty for failure to exercise common prudence and skill in the performance of a fiduciary's duties.
(obsolete) An excessive load or burden.
(law, obsolete) The putting, by a commoner, of more animals on the common than he is entitled to.