"Additional payments" refers to extra money that is paid in addition to a regular payment or agreed amount. It can be an extra cost or an additional amount given for a specific purpose. Full definition
I just went through the same argument with a colleague who was making additional payments of $ 1500 per month into his mortgage. (affordanything.com)
This is a request for additional payment for something you are currently able to do for free, and doing as part of an existing contract. (law.stackexchange.com)
You might need to make additional payments of tax if you make a profit. (money.stackexchange.com)