This is a little bit apple's fault - if any app is distributed outside the app store, the developer now has to pay a fee per-install even for installs from the app store (50¢ per install!). He could have chosen not to put Delta in AltStore and only put it in the app store, then it could be free on the app store. Either way, you don't get a choice. You install from the app store, or you install from AltStore, but it can't be on both because of apple policy. This is what apple wants, and is why the fee exists. Hopefully it violates the DMA and they'll be forced to change that.