Just curious. First, I gather that all the pkgs (especially USPS) had tracking enacted so you could follow them (theoretically). As for the merchant that "did not comply," it could be that they had an "exclusive" deal with USPS (ie. cheaper rates if you only ship with us). I used to have problems with UPS. They refused to make it easy for me to "pick up" a shipment at their distribution facility Their mantra was, always, "DELIVERY.... DELIVERY.... DELIVERY!"... at a time when I was always at work and never home to receive the pkg upon initial delivery), I was asking the merchants to ship USPS (oddly enough)) which would would go to my PO Box where I could pick it up securely, or FEDEX where I actually worked nearby and they *would* hold the pkgs at their office for my pickup. Most of the merchants I spoke to using UPS exclusively told me that they had cut deals with them for cheaper rates if they did use them exclusively.
Later on, when UPS had that huge and long strike by the workers, a lot of those exclusive deals started disappearing as the merchants started "diversifying" their carrier strategy. I guess that's when USPS started to get really big.. apparently, too big for its britches. It "Peter Principled."
And I'd never do business with that merchant again... and tell them why... on both "Glassdoor" and "Yelp!"