[HELP] Royalties payments

Hi everyone,

My partner has an audiobook gig set up on Fiverr. We are pretty new to the website and we need your help with something :

One client needs 3 books narrated and offered to pay royalties on top of the original fee.

From what I read, Fiverr has no system for this and the payments for royalties will have to be done outside it.

Does anyone have any experience with this? How do you track how much the payment will be? Did you create another contract especially for this?

We appreciate all help.