Hey :-) your post is already older and I don't know if my answer still helps you. But maybe for others that are also looking for it because I found this entry on google. I have released an app with BuyMeACoffee as link that is opened in an In-App-Browser and it was successfully approved from apple. Now I added BuyMeACoffee to a second app, but here with the possibility to do a monthly payment (Membership) and this app was rejected. I removed the membership and try it again now. Keep you updated.