USPS transfers to Canada Post, so in fact it is Canada Post that is choosing not to charge you terrible fees like the other ones.

But yes, always use USPS to ship to Canada.

I have stopped ordering a physical product from the U.S. more than once when presented with UPS as the shipping handler.

If you open a UPS or FedEx account, you can usually bill the charge to it which removes a big part of the handling fee, so you only pay the duties+taxes and possibly a small surcharge (nowhere near 40, I think it was, max, 5 when I used to use it).