This is the can of worms introduced by doing event processing. As soon as you break the ties between request and response a billion questions come up. Asking what happens to the reservation in a request response scenario is just chuck an error at the user and ask them to try again.

As soon as you accept the user input and tell the user all is well before you have processed it you enter into these kinds of problems.

I know not every single thing we do can be done without this kind of async processing, but we should treat these scenarios more seriously.

It's not that it can't ever be good, it's that it will always be complicated.