> And let the user know they can purchase outside the app for a discount.

I expect that's not allowed either. Look at Audible. There is nothing that tells you that you have to sign into the website to buy books, because Apple won't allow it, and Apple will take a 30% cut if Audible does it in-app. They rely entirely on users having prior knowledge that (a) you can buy books at all, and (b) the app is just a player, and the store is on the website.