Apple’s own rules

3.2.1 Acceptable

(vii) Apps may enable individual users to give a monetary gift to another individual without using in-app purchase, provided that (a) the gift is a completely optional choice by the giver, and (b) 100% of the funds go to the receiver of the gift. However, a gift that is connected to or associated at any point in time with receiving digital content or services must use in-app purchase.

How they interpret these rules when it comes to zapping notes: