Good point, but I'd add that not only when charging money, but any operation that can have side effects if executed more than once (emails, external calls, etc.

Good point, but I'd add that not only when charging money, but any operation that can have side effects if executed more than once (emails, external calls, etc.). Having idempotent queue consumers/listeners can help with overall systems resilience (someone already mentioned this below). It helps as much as you don't have to worry when restarting a queue broker and/or apps with consumers. It adds complexity, but I think it pays off in the long run.