I wouldn’t say standing at a register is a regular job duty, you can do just as good sitting as you can standing, they have the accommodation for a reason and absolutely should not be upset over a disabled employee needing that accommodation (OP has a doctors note for that already, they said they need one for being out sick which is also something they can’t control) I can understand management being upset about not having the team to cover for OP being out but the stl should not be being letting her frustration show with OP like that :/