You'd have to contact Chit Chats or Stallions with those questions, or an actual customs broker. I genuinely have no idea.
I suspect if you are sending via the postal service it would just get refused by USPS if there were duties owing. Zonos does have an option in the app to add a receipt, which might help show that it's a genuine return. USPS apparently is not set up to collect the tariffs, so I suspect anything where the tariffs were not collected as they should be would be RTS.
My assumption, just going off pure logic, is that if Zonos pulls up no tariffs and those codes are as they describe, people would be fine to return goods with Canada Post. I would imagine that if someone were to send a large quantity of packages to the USA using those HTS codes where it doesn't like like regular consumer patterns, CBP might look into it if they have any level of enforcement for senders misrepresenting customs info.
With that said, most US purchases at least on eBay come via eIS. That eliminates the US seller for returns. So from that perspective I assume most US returns would be fairly seamless.
I also don't think this will be a major issue, because how often do you have to return something? I can't speak for anybody else, but I think I've opened like one eBay return in the last decade. So I'm not overly concerned about having to return stuff to the USA.
