I can't believe it because USA 1 is cheaper than the LMC truck that you buy there? I still do not understand
No; they do have some stuff that's nice quality (if you ever get it), and offer some parts/packages that other companies don't.
For example; when I wanted door panels for my Burgundy '87 Silverado, LMC only had black (or tan or blue) for the door panels. Some of the insert pieces were dyed, some weren't (they were black.) They did not offer assembly.
USA-1 offered complete door panels, assembled, in SEM matched burgundy. They were about $20 more than me buying all the pieces (half in black), having to dye them myself, and assemble. They even would add window/lock switches to the package for me.
They offer convenient, and competitive, prices for that stuff. If they shipped in a reasonable time, I'd love them. Between them and Orange Country Truck Parts (who has since changed their name), I had a few thousand dollars in promised parts that took forever to arrive. One truck I was working on for a client, I *forgot* Orange Co still owed me some plastic underhood insulation screws/clips. The truck was gone (completed, done, checks cashed) for near a year, my American Express got hit for $15, and box of a handful showed up in the mail.
I legitimately didn't know why they were there. Had to think about why/what happened. Still have them. AMEX did a chargeback.
The dirty secret is LMC makes most of the Square parts on the market. They sometimes get repacked. Same is true with most cars out there (Corvettes it's Paragon, for example.)
I'd avoid those two vendors like the plague; I've had shipping delays on every car I've restored. Most companies communicate, and explain why on the occasional order. These two are bad.