Learn English – “Accountability over” or “accountability for”

Hundred by itself does not constitute a quantity. You must have a number in some form:

I'm one hundred percent sure.

or something else to provide a number — like "a" which denotes a singular quantity.

I'm a hundred percent sure.

Without a number it is similar to saying

I have milliliters of water. [How many milliliters?]

I've used the phrase myself before and heard it dozens of times in all kinds of contexts. It is correct, it is in common usage, and no one is going to think it sounds funny.

However, from a resume perspective, please don't make supplementary income the focus of "reason for applying." Nobody who is looking to hire wants to read that as the #1 reason, even if it's true.