Sometimes things do not work as promised
I've been having some trouble with my credit card reader lately. It's been balky, and a few times this summer, it hasn't worked at all. This is new because for years it was nearly perfect, even taking payments when there was no cell service.
To be honest, most of my clients are wonderful, respectable people, so if it doesn't work, they're happy to just pay an online invoice later in the day.