For me, I only ship USPS because thats whats convenient for me. The nearest fedex/ups pickup is two towns away and almost 10 miles from me. USPS does offer Canadian shipping but has some down sides.
#1 Cost. Its through the roof. Ontario canada is obviously closer than California but its cheaper to ship to cali. Like a third of the cost cheaper
#2 no tracking. USPS does not allow for packing tracking in canada. In the us i know when and where everything was delivered but in canada nothing.
If the usps would offer better shipping arrangements i would ship to our northern neighbors in a second. money is money!