This is the 1st this has ever happened to me bt good to learn some people also had trouble. and since you said it was mattel that is almost the same thing since the smc weapons are meant for MOTUC.
Probably what happened to your package even though it was a return was probably the carrier at the time realizing what he had in his hands and stealing it for himself, probably to sell on evilbay, and messing it up for you and mattel. I just hope this hasn't happened with my stuff, I know their are a lot of SMC fans out there.
In my years of having toys shipped to me, though, that's the only issue I've had. I'm much more liable to think it was someone at Mattel/DR's warehouse. Nothing was messed up for Mattel because the extra order was my fault.
Unless your carrier knew specifically about the SMC items, I highly doubt that was any malicious intent.