By far, the cheapest way to ship from the US to Canada is USPS 1st Class Mail International. You cannot print postage for this option online, but can bring the package to the PO to have the postage affixed. You'll need a customs form completed, but this is a VERY cheap way to go.
If you need trackable shipping, USPS Priority Mail International (to Canada) is still affordable... and the flat-rate envelopes are quite the bargain.
FedEx or UPS. You could hire a less expensive carrier but if they lose your package, they will become the most expensive. "Global Shipping" is FedX certified. Check their prices.