The phrase "extra amount" means an additional quantity or number of something that is more than what was originally expected or required. Full definition
The only drawback is that you will ultimately be paying considerably extra amount of money over a longer period of time. (badcreditloanservices.com)
Paying extra amounts to reduce your principal ahead of schedule can help you pay off your mortgage faster and reduce the total cost of interest. (valuepenguin.com)
Monthly extra payment: Extra amount added to each monthly payment to reduce loan length and interest paid. (interest.com)