Is there some kind of standard signage for PWD to be allowed to sit at cash?

At my job, we get in shit for having a stool behind the counter to sit on occasionally but im on disability for a few reasons but one I have a bad back and my boss says she gets in trouble from the higher ups because undercover shoppers rat us out. How is this allowed though? I'd like to put some sort of sign up to just stop this from the beginning but i dont know how to Google that. I just keep finding priority seating signs.

In Canada.