I received the package today with a charge of $12.94CAN. This is $3.70
Federal Sales Tax plus $4.24 Provincial Sales Tax plus --- wait for it ---
$5.00 service charge!!

So now I have paid $16.25US plus $12.94CAN via the postal service versus
$21.49US for UPS. I don't know if there would have been additional
charges
by UPS (taxes etc). Perhaps someone can enlighten me on that point.

You have to pay the taxes anyway regardless of shipper. The $5 service
charge is a bargain; UPS brokerage fees would have been many times more than
that.