National Shooting Sports Foundation (NSSF) press release [via ammoland.com]:

National Shooting Sports Foundation (NSSF) this week learned that United Parcel Service (UPS) had changed its policy regarding the shipment of firearms suppressors. New policy states that UPS will no longer ship suppressors, even between licensees. NSSF is working with UPS executives to determine what prompted the enforcement of this unwarranted policy. We are unaware of any thefts or losses that would explain the shipping company’s sudden decision to enforce a prohibition against shipment. NSSF will keep you apprised of developments. Separately, NSSF is also working with the U.S. State Department to achieve a change policy to allow export of suppressors.