I think he wants the best price on his airsoft internals and as far as I can tell he has no problem replacing the externals. So really he is hoping that the visit to his cousin will become an airsoft spending spree.
IMHO get your internals from washington and declare them at the border, while you drive home do some math and figure out how much you saved on initial cost. Then do the dollar exchange and add the fuel costs. Buy your 'Canada ready' externals and calculate your final cost.
That price is why I won't be in the states following your lead but as far as you going to see your cousin and grabbing some hobby stuff, well I do it all the time.
PS If you are not already visiting the states anyway, see other replies.