I just got a reply back from Airsoft GI confirming what everyone except AMS (who was wrong) has said:
Thank you for your inquiry. AEG metal bodies alone are not considered compliant with CBSA regulations. I apologize for any inconvenience.
If you have any further questions, or if you’d like to place a phone order, feel free to call me at (909) 869-0671 ext. 128 during regular business hours (9:00am – 5:30pm PST, Monday – Friday), or email me at firstname.lastname@example.org. Thank you.
I'll stick to ordering within Canada for parts like these; hopefully Huang at AirsoftDepot.ca is able to restock M16/M4 AEG metal bodies at his shop sometime in the near future, because I'd be very interested in placing an order for a reasonably-priced metal body (like the $99 DBoys M4 body). I'm a bit concerned that he might not be able to get them in, as he was unable to provide an ETA the last time I was able to contact him.