Royalties are typically a contractual payment, though. Calling a voluntary payment a royalty is kind of an odd use of the word, if not plainly incorrect.

You're right, these are donations. But they are donations that are determined based on the number of units sold. I believe that's why the parent is using the term royalty.

It's not a contract, as the term "royalty" implies, but it is a royalty in the sense that it's determined per unit sold.