Need a seat belt buckle and a few places have it for $30. It's about 6 or 7 inches long and easily fits in a Priority Mail $8 flat rate padded envelope and includes tracking. One place wanted to charge $30 to ship UPS. Another said $20. No thanks. Might as well get new from a dealer for $66.
Those salvage yards would likely pay $30 just to not have to deal with selling you a $30 part.
Incorporating USPS into their shipping system would require extra time and money, and if they are only selling $30 parts to do it, it isn’t worth it.
They are a business, and the prospect of losing money just to make a few customers happy won’t motivate them to attempt it.