How did you come up with 80%? Have you tested before? Pushing carts seems fine as long as they are not too heavy. I wouldn't really lift anything if it seems too heavy, just to be on the safe side. Once you find out for sure, I would notify your boss and see if they can adjust your working conditions if your dr. is not comfortable with it.
you show be fine doing that, just dont overdo it. when ya plan on testing again?