Explanation for folks:
The UPS store is a strip mall business (it is not UPS) so it doesn't fall within the legal exceptions for shipping from a private party to a FFL. Another way of saying it is, the UPS store is not a common carrier. Private parties are permitted to deliver firearms to the common carrier for direct shipment to an FFL. All gunbroker (or these others, apparently) are doing is an intermediary to print a shipping label, e.g. like stamps.com, merely with a gun themed website and relaying some of that information. But it's no different if you print out your own label directly.
I'd recommend dropping it off at the hub for sake of insurance etc. in case something goes south.