Possibly you got shipped the wrong item. Otherwise, it's just CBSA fucking with you cuz it's airsoft-related. They have the latitude to make judgement calls any way they see fit, and if you feel it's wrong you have to contest it (which is what you need to do now).
I've seen this happen a few times, and often the package is just released without any explanation or resistance. It's like they just wanna hassle you. Or maybe the dude's got a quota to fill, I dunno.
Either way, a mechbox is in no way either A) part of a firearm, restricted or otherwise or B) a replica, or receiver of a replica.