What is one figurative shift? Doesn't make sense, does it? There's no reason to use "literally." You can use a stool on your days off.

I always found it silly they don't let checkers in the US sit down.

Perhaps it’s still too soon to return to work. Or inquire before the start of the next job if use of stool would be okay

I had the same problem after my back surgery. (6 pins, 2 fusions, and a cage). I let them know ahead of time that I can't stay in 1 position too long, I must rotate between standing, sitting and moving around. luckily, I found a position at a restaurant as a cashier, (with a stool), and a once a week 3 hour shift as a lunch waitress. Some people think after back surgery your back is cured. It's not, it just makes doing some things tolerable.

Next job, check for Reasonable Accommodations prior to accepting.

If you didn't tell your employer before you started, then YDI for not giving them fair warning. If you did and they still fired you, lawyer up because that's discrimination on medical grounds.

In the US they’d likely have to give your reasonable accommodations under the ADA a stool would likely count. Not sure about Canadian law.