Yeah I bought a new quartz(!) Tag F1 from an AD and it lost time when I wasn’t wearing it (really, I took it off at night and in the morning it would be slow but kept time fine when I was wearing it).
I took it back and they swapped the battery. Did the same thing the next night.
I said I wanted a new watch, and they said they would repair it. We agreed a compromise that they had two weeks to repair it I think, and if it wasn’t fixed in two weeks they’d give me a new one. I got the new one without any further fuss.
When you say “cheap”, what kind of budget have you got in mind? What’s the total number of users, you said 5 users per shared mailbox, but are they all different, i.e. 5 users x 5 mailboxes = 25 users? Or is there some overlap?