A mechabox is just that.... a mechabox I don't know why it would get stopped. (If there's a new regulation on mechaboxes then I might be in some trouble this summer...)
Try to dispute it. It's not like it's a receiver or prohibited. DONT GET MAD at the customs guys just explain it calmly and in a professional manner. They might be more inclined to let it go if you're professional and polite about it rather than you getting irritated and chewing them out. They're just doing their job even if it was wrong that they confiscated your gearbox.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE