The Sentry extension turns out is a pretty bad example of extensions and I hope the future extensions are much better.
It doesn’t have any integrated billing, so billing still happens with the 3rd party. You lose the “free tier” if you want to go over the included limit and want to pay for the overage. It pretty much does nothing more than create an account for you and set a secret.
I would expect extensions to be worthwhile integrations with billing happening through fly and not being a glorified signup process.