If I recall correctly you've been refusing to see a medical doctor or chiropractor for the issues you have. Honestly the only way you'll convince an employer you have an actual medical issue barring you from working a normal shift is with a legitimate doctors note. If you want your short shifts then I advise you to seek medical care for your issues and get documented proof to pass on to your employer.
That takes money, and, frankly, once you go to an employer with any sort of medical problem -- especially back problems -- you most likely WILL be fired and an employer can always find a reason. You will also have a very, very difficult time getting another job.
That is the reality of how things work on this side of the border, I'm afraid, and it's not speculation or talking about something I don't know anything about, that's from someone who's not only been in the workforce, doing retail as well as working in a factory (for awhile as a temp, and trust me there is nothing lower on the totem than a temp factory worker) and as a paralegal handling all manner of workers' comp and workers' rights cases in state and federal courts.
Retail employers like to hire students, especially, because there are a lot of them, they're young, inexperienced and won't rock the boat (usually) and are malleable and don't know what to expect, what (very few) rights they have, and are still young enough that most haven't shaken off the mindset of being disciplined or run over by authority figures.
If you're able to wear comfortable shoes in a retail setting, DO IT, like everyone says. I worked in a music store where we were expected to dress like we would in an office
Thin carpet over a concrete floor. Even flats didn't help. Dansko or Born clogs can be your best friend
There are a lot of trade-offs when you're starting out working, and it can be hard, working through them. Venting helps