How can I track revenue from my game to pay royalties?

I’m new, and I want to make a gmod killer with better physics in a better engine and a larger gamemode potential. When I release the game (most likely on Steam), how do I track revenue?

I want to release the game for free, with a donation box in the corner as a “pay what you want” kind of option. How would I track those donations, and how would Epic Games tell if I’m lying on my revenue report to pay royalties? Would I possibly flag a false positive in their system with this setup?

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