Working in retail myself, one big issue is that these tasks are an everyday part of the job that will simply never go away, in some jobs you can come back on 'light duties' where you can avoid most manual stuff for a while but in smaller fast paced places it's sometimes just not an option.
IF you're off when you come back you should have a back to work assessment or talk of some sort where you're asked what happened, whether you've fully recovered and what adjustments if any you feel are required (more or less) and that's obv your best chance to make it clear you struggled with that particular task.
I've been in the exact same boat where loads of repeat lifting and carrying gave me a sore lower back, which has led to me shifting my weight funny for a few days then twisting funny which has then caused a greater nerve issue in one leg (sciatica) which was maybe the worst consistent pain I've ever been in, yep that was me off sick unpaid for 3-4 days and still in milder pain weeks later.
Not that I know what your leg issue is, but I only got relief after trying a number of specific stretching exercises and finding one that flexed my nerve in just the right way to relieve the tension and let it recover. If nothing else if you're going to be in this sort of work it might be worth consider a series of stretches or even yoga etc to keep yourself flexible, it really does help. Decent knee pads or something to kneel on can make a lot of difference too, I used to use a small doubled up rectangle of yoga mat foam just to keen my legs from locking.