Amy, how much of a difference is each side? Maybe just putting something thin under one end to level it out. I was thinking something like a paint stick. Those are fairly thin and wouldn't be such a pain if you needed to cut them down a bit. You could even stack a couple on top of each other using duct tape. IDK, just trying to think of something that is fairly cheap and easy. :)

I've occasionally done that kind of thing. But many different shelves. Some indoors, some in greenhouse. I was just frustrated because I lost some seedlings. I thought maybe someone had a solution I hadn't thought of. I want a floor in my greenhouse this fall. Maybe that would help there any way. Indoors the carpet screws with me.

I probably would shim the flats if the shelves are uneven. You could use anything from actual shims to paint sticks to popsicle sticks.

Could you use shims to shim up whatever side of the shelf unit is too low? That might be easier than shimming the individual flats.