As Mac Jim ID has described, this is the new scam, thanks to the EU forcing Apple to allow side loading. Apple has no control over pricing, or even having all pricing disclosed as they do in the U.S. App Store.
And it's not just the EU. Recent changes to U.S. law also allows vendors to handle their own charging outside of the App Store where you may have no idea what charges may occur after acquiring the app.
Like from this recent topic on these forums:
About a week ago, I took an IQ test through a website called IQBrain. To view my test result, I was asked to pay $1.98 via Apple Pay, and I made that payment.
However, today I was unexpectedly charged $28.80, and I never agreed to any subscription or recurring payment. When I checked my Apple ID subscriptions, there was nothing showing related to IQBrain or any active subscription.
This charge does not appear in my subscription list, and I cannot find a way to cancel or report it directly through my iPhone.
This one was completely outside the App Store, but (apparently) never disclosed there would be a subscription fee. The only way to get out of their hidden subscription is to manually tell them.
https://iqbrain.org/terminate
And how many users are going to know to look there? The options are in plain view on their lead page, but you have to scroll all the way to the bottom to find the only notice to unsubscribe or request a refund.

Expect this type of undisclosed (or very well hidden) type of charge to multiply as vendors put more of their fees off of the App Store. There will be no unsubscribe option to find on your App Store purchases as they were added directly by the vendor to your credit card.