I have had discussions with Canadian buyers about shipping from the US. I just got back from the post office and learned a few things.
The US Post office will not ship a coin, even in a cardboard holder in a #10 envelope without a customs declaration. The letter is thick enough that it is designated a Package. I told them it was a thick brochure, they knew it was a coin.
They would prefer shipping the same coin in a bubble envelope. Said it is safer.