Say something is coming from China; if it goes through Amazon, they ship it to a US warehouse (import fees) and from there to a Canadian warehouse (more import fees). If they ship it by post, it goes from China direct to Canada, never touching the US, and both countries have de minimus exemptions, so on small purchases no fees are levied.

Eventually Amazon will get their supply chains adjusted so that non-US stuff never crosses the US border, but they’re not there yet.

Same thing happens for FedEx and UPS; for FedEx, stuff from China flies to Anchorage, from there to Dallas or Memphis, from there to Windsor, and from there to wherever you are in Canada. For UPS, it goes to Ottawa California, then to Delta BC, then to somewhere in Canada.

As I said, eventually the shippers will get this sorted; Prince George BC will likely become a major shipping port if these tariffs last for very long, and shipments will stop touching US soil when they don’t have to. But until they do, the postal service is one of the only carriers that isn’t transiting the US between other countries. And even when they do, USPS doesn’t have to pay anything during shipping, so no tariffs are applied if the package isn’t destined for the US.