I would choose not to use the app unless there was very clearly value for the cost.

If a service was literally doing as you say, charging more through Apple's App Store vs their site, I would choose not to use that app. At all.

Part of the real appeal is the pro-consumer parts of the App Store. Like I said, being informed before renewal is something that very few services do but Apple does for any subscription.

Edit: to be clear, I think even $99 is too expensive for Hey.com. This is my opinion. Some people may find value there, but I don't think Hey does enough to justify being more expensive than Fastmail (which also has important features, to me, that Hey doesn't).