The cheapest (Shippo charges for non-eBay labels) is Canada Post's Snap Ship.
https://www.canadapost.ca/cpc/en/business/shipping/find-rates-ship/snap-ship.page
You will need your Solutions for Small Business membership ID and password for the Canada Post website so you can get the discount. You will also need a credit card to pay.
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The shipping calculator can show you the cost:
https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1
If you are not signed in you will see the over-the-counter rate.
For what you could save select "Show minimum online savings" on the 2nd page.
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Some example USA savings (500g package, ON to CA, SfSB level 1):
Small Packet: $11.52 vs $11.15, save 37 cents or 3%
Tracked Packet: $19.47 vs $18.25, save $1.22 or 6%
Expedited Parcel: $22.38 vs $20.65 save $1.73 or 8%
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