Rnee, maybe have a chat to them and say that you really enjoy the work, enjoy working for them, and are committed to the job BUT due to an injury, is it possible for you to sit on a chair at the register. Explain that the injury does not impact on your role as you can still reach for products, scan them, put them in bags, and lift bags - you just can't stand for very long periods (if this is the case). They should be able to accomodate you. I have seen plenty of check out operators who utilise seats in their work.
Otherwise, have you looked into a desk type job?

HTH

MG