How would the driver know the contents, even if Glock did declare the package contained a gun to FEDEX?
Based on packages I've received with items missing even though they were on the shipping list, I believe drivers learn certain things from return addresses and labeling. E.g., ammunition and gun parts (scopes, e.g.) will often have a sticker on the box with " ARM ORD" on it. Packages from places where I buy gear and cutlery have twice been robbed of the cutlery - like a CRKT M-16 folder - with the original box put back inside. It was done once through a box that wasn't taped on the bottom, and the the other time through torn tape after the box had apparently been 'dropped'.
So even if the return label says "Aunty Em" on it they eventually know the address is a vendor of handguns, or custom gun work, or knives. And drivers and handlers do steal - if they don't want the item, they can sell it. That is why if you call any of the vendors and tell them your box arrived empty they will simply send a new order. Companies like UPS and FedEx pay for the replacement without even an investigation, which suggests to me it is too common for general safety. Even if they pay to replace your handgun, when the one you bought the first time shows up at a crime scene, you're getting a SWAT visit.