Pay $10 at USPS for their extra special separate fragile handling and the box filled with expensive electronics still arrives banged up.
And every box size for priority if it fits it ships mail has two or three separate dimensions. So you can't just get a box that says large, because there are multiple versions of a large box. And some they keep behind the counter. So if you go into the PO Box portion of the office thinking you can just grab one of the boxes out in their cubbies, nope, you have to come back during business hours.