As a seller who gets burned by this as well, I can shed a little light.
First, some of that is ripoff "brokerage" fees and some is duty/taxes. I'd say about 85/15 in general. And it does appear that USPS gets a significant break on duty/taxes charges for small consumer goods. So even if you remove the ripoff fee's it's more advantageous to use USPS.
Second, the seller has no way to know what those fees are until the customer calls complaining. I can guesstimate that one average they will be $25-30 USD (plus duty/tax) but that doesn't include exchange rates, which vary widely.
Also, the seller can elect to be billed for the "brokerage" fees. Presumably this is to allow shippers to charge those fees before the sale, but why would you ever do that without knowing the amount due upon completion.
So why ship UPS at all? For starters, USPS tracking SUCKS ROCKS. About 35% of domestic tracking has zero data, and 100% of international has no data. UPS on the other hand tracks 100% door-to-door.
To make it worse, USPS shipments are not insured and even if you pay extra for insurance they do not pay out until AFTER they have determined it is lost forever. Which can take months on an international shipment. UPS, on the other hand checks tracking info, runs a tracer, pays the claim.
Also, USPS flip-flops between picking up international and requiring it to go to a post office to be accepted. Some days it goes just fine, others it sits around for a few days before being returned to you with a note. Whether or not it goes out or not depends on the carrier, their supervisor, the local postmaster, the local airport office, and homeland security. If any one of those changes (or has a substitute) your packages may be returned to you.
Finally, USPS will not carry any material labeled as ORM-D. This includes any aerosol product, combustibles (like pump hair spray or nailpolish), etc. Which means that a large portion of consumer goods must travel via UPS or not at all. For us, it affects about 30% of our product catalog.
That said, I agree that UPS to Canada sucks for sellers and buyers. But USPS isn't any joy for sellers either. If anyone can come up with an good alternative that solves the above problems, please PM me.