I can tell you that as a buyer in Canada, I would never knowingly choose UPS. They have a sliding scale of brokerage fees based on the value of the package, and the rates are quite high. For example, lets say I purchase a small set of chips valued at $550 CDN. I'd pay $10 brokerage on any shipment greater than $20 CDN if using USPS/Canada Post, and that's all of the fees. For the same set of chips, I would have to pay UPS $68.50 in brokerage and a bond fee of 2.7% on any customs fees and taxes owed (or $6, whichever is higher...the breaking point is $223 in duties/taxes owed). So it's another $6 to UPS, as we have to pay sales tax.
For the exact same shipment
USPS/Canada Post = $10 in fees
UPS: $74.50 in fees
For the exact same shipment
USPS/Canada Post = $10 in fees
UPS: $74.50 in fees