Fixed Sum, Flat Rate
noun, の adjective
A determined amount is an amount that is fixed, aka a fixed amount, a fixed sum, or flat rate.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
I changed it to the flat rate plan.
WaniKani is available for a fixed monthly amount.
Um, I’d like to open up a fixed deposit saving account.