Their fee model is not completely deterministic / predictable which makes it a bit hard. They say if you assume a 0.4% fee for incoming payments "on average", you should be ok but I'm skeptical / I haven't grocked it yet.

A Cashu mints needs to know exactly what the resulting balance and the fee for an incoming or outgoing payment is going to be (or at least know the maximum fees, to be able to refund overpaid fees).