USPS (United States Postal Service) contracts out the international shipping. Packages will get trucked to the partner location via USPS. Once it enters the gate, it’s under the control of the 3rd party contractor even though the USPS tracking number remains the same. The cargo is held until the truck or plane is full enough for a certain destination.
We’ve stopped shipping internationally via USPS because of the slow shipping times, unreliability, and poor/zero customer service. FedEx International Ground service has the best delivery service from US to Canada. Although the shipping cost isn’t expensive, FedEx sends an invoice to the importer after delivery for taxes and fees. It’s always surprising to me how much they charge. On the other hand, DHL is very expensive to send internationally but I don’t know if they send an invoice for taxes and fees like FedEx does.
Can confirm re: DHL. They do the same things that UPS and FedEx do with regards to the taxes and brokerage. At one point UPS had some level of service that included the brokerage in the shipping cost. So while that service was more expensive, it was less than what you paid with UPS Grounds + $30 brokerage + taxes and they sort of take advantage of that by not being super transparent about it (most people pick the cheapest option unless they are in a rush).
In my experience with USPS, Priority mail is pretty good, but USPS ground is not ideal.
Look into Pirate Ship as a portal to UPS or USPS. Time saving software & great prices. A third of my sales are international, The last 3 days I sent tools to Wales, Australia & Scotland. Customers often specify USPS because the private shippers make sure the package goes thru customs, then charges a fee for doing so.