Nice she-shed, Willis.
Welcome to the world of turn-key storage buildings.
I'm in the middle of working on mine, and would like to add some items to watch for.
Yes, ground prep is very important. Flat/level is the best. And for those in rainy areas, who actually have genuine soil, strong recommendations for a bed layer of 2-4" gravel.
If you don't want direct ground contact, placing it on 2" thick pewter stepping stones works.
The catch is, getting it leveled. Properly. By the right people. With the right stuff.
I'm completely re-doing the leveling on mine.
I was too hot and tired by the time these guys blocked and leveled mine, and didn't catch their shoddy work until later, after signoff.
Here's some horror pics.
You can see daylight beneath the unsupported skids, or a tiny stick to fill the gap!
They only blocked the outer periphery. Looking under, you can see absolutely NO blocking, NO support, at all in the center section.
I wanted this to be as low as possible, opting for direct ground contact. In re-doing this placement, you need to jack the thing up at one end, get in there somehow, and fit or remove blocks/shims. The finished building is actually a very rigid box, no worries about twisting. But, to jack up an end takes some doing. Low clearance, and needs more than your simple auto hydraulic jack. My 2.5 ton jacks won't lift it, had to use a 6-ton bottle jack. Best would be to rent a toe-jack, but need to enlarge and reinforce the lift points.
You don't want to get involved in doing ANY of this nightmare. So, good ground prep, and watch them closely.
My building was well built, to better specs. (4) 4x6 treated skids on 5' centers, 2x6 floor joists on 12" centers, 3/4" flooring, studs on 16" centers, roof trusses on 24" centers, 7/16" roofing sheathing with 29ga metal atop that, 6" eave overhangs.
The substandard stuff out there will have thinner flooring, wall, and roof sheathing. Joists and studs spaced out to 19" and 24". Sometimes even using 2x3 instead of 2x4. No eave overhangs. Just things to watch for.
I just noticed that my window is missing the screen. Not easily found, need to either custom order one, or make it myself. *groan*