And if you really want to oversimplify the discussion to a brick and mortar analogy, it's common for grocery stores to make taxes explicit in receipts: https://i.imgur.com/Lp3nwlH.png
In the app case, you bought an IAP from Apple (I know it’s Facebook’s app, but that doesn’t change the fact that the transaction was executed by Apple) but somehow think it’s ok for there to be a note saying you can pay less if you buy it on your computer instead.
It didn't say "go to this other website and just pay $7". I think it's totally reasonable to disclose the fees, I would even argue that it's the morally correct thing to do.