All this talk about using the USPS board game box to ship an intake manifold is comical, just remember: you may expect a seller to go that route, that you should only have to pay that flat rate fee, but the seller decides.
You want to “in the eyes of the USPS” commit fraud by using that box for something other than a board game etc that’s your prerogative, I don’t give a rip, but the key point is: keep in mind you as a buyer have no recourse if it’s lost or somehow gets damaged. And you can’t insure it. And as a seller, what you gonna do for the buyer at that point?
I’ve had a few buyers (as well as inquiries) of intakes I’ve sold ask about using that box. Nope. The cost is what it is, and it’s insured for the actual selling price.