Can you turn off one door while the rest of the system is active?

Unfortunately, you would have to do it manually in the phone app. You could possibly consider using the different modes to get this done too, home and away but may just be as easy to use the keyfob and have your mom arm the system while out getting the mail and just turning it off to get back in, and then turn back on again.

Why not just use the key fob? Push button to disarm system, walk outside to get mail push button to arm and same when return to residence?