In my past experience, UPS will charge you an exorbitant "border brokerage" fee to bring the parcel over the border. Like you, I will only order from the US if the vendor agrees to ship via USPS. I've not had a problem with getting a vendor to ship USPS if I specifically ask for it. If you must agree to shipping by UPS, be sure to contact UPS first and find out what they are going to charge you for brokerage fees - you may find that it is not such a great deal after you add those costs.
Anything worth doing is worth overdoing.