When thinking of ordering weapons over the border in simplest terms look at it this way. A RS M4 rifle uses a 5.56mm round, an Airsoft M4 uses a 6mm round. Now imagine how close in diameter the barrels are. Now imagine a customs agent unfamiliar with airsoft and sees many packages a day and can't stop everything to check that the barrels are about half a millimeter different in size. That is if he/she takes it out of the box, most likely it will be X-rayed and if it looks like a weapon which amazingly it does *poof* gets tossed aside.
P.S. none of what I said may be accurate but when I first started looking at guns and saw ones from USA here was my first thought "This looks pretty damn real, how the hell would this get cross the border."