That's definitely large BS.
The UPS Store, whatever, is a franchise. You not only can't ship everything you want, but you pay a higher rate than if you deal directly with UPS via their "customer counter". Unfortunately, customer counter locations aren't very convenient and few in number. This is why when I ship a firearm, I go to the UPS customer counter.
Also, I ship my triggers via USPS Priority Mail and of some 2,000 plus mailings have only had 2 problems.
When I want a ground carrier I use FedEx at Kinko's which is direct business with FedEx.