USPS only. IMO, the other two are just asking for problems.
Buy the book" Legal Muscle" from Rick Collins (available in our bookstore). He goes into detail about why. But basically it comes down to the fact that UPS and FedEx can open any package at any time they want, with no reason at all. The USPS cannot, because they are a govt entity. IIRC, it falls under illegal search and seizure.
Bottom line, dont send a package unless its USPS.