Vendors always use the US postal service. Seems odd, although satisfying, to use a government agency. Is it because it's cheaper than a private shipping company like FedEx or is there another reason? Wouldn't the authorities be more likely to inspect packages sent through the regular mail?
UPS
Page 15, Official UPS Terms of Service
FedEx
Official FedEx Terms of Service
USPS
US Postal Inspection Service
So, only USPS provides protection against inspection en route from law enforcement, and of this only First class and up qualify for this enhanced protection provided by the 4th amendment. So only these special packages need a warrant or probable cause (drug dogs). That's why vendors use USPS. :D