Not sure about your resin tool shed, but I used to work with a large machine used to chemically etch printed circuit boards, made of heavy pvc sheet, and we ran the heated sump at 130 degrees F. I would expect an outdoor shed made of plastic resin to sag, eventually.

FWIW, I have a plastic resin shed that I use for storing gas and other flammables outside my barn. It's the tall skinny one you've seen at HD, Lowe's, etc. I think it was originally intended for "stick" tools. Anyway, I added shelves for gas cans and the like. I'm in SE Ohiio and in recent years it's been 100 fairly often in the summer. It has never sagged or deformed.

Haven't dealt with HD in years as there's not one around here. If they're comparable to Lowe's, Lowe's has product specialists that will answer your questions or immediately put you in contact with the manufacturer. Doesn't hurt to ask.

Good luck,

Ev