Howdy Jaaks, quite the huge wad of money you put with ehobby. Hopefully that works out for you without issue. If you didnt order anything restricted like fake founds or an AEG you should have no issues at all. However, it has been noted that customs can be a little over curious and take mags apart and not put them back together. I'll stop there though, I dont want to put any scary thoughts into your head.
As for your cousin, unfortunetly he will have to leave his gun in the states and rent one when he comes to Canada. He can always take the chance and try to get through the border either sneaking it or attempting to declare it, but honestly I would chance neither and just rent one when he got into country.
"Its only a little bit on fire"