Start here:
https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1
Use metric measurements. Canada Post went metric in 1974 and everything else is guesswork.
We get discounts from Canada Post prices when we buy shipping labels through Shippo/Paypal or SnapShip/Canada Post.
You will need a Small Business Solutions number, but you can get that at the PO or even on that website. It's free.
Eventually a card will arrive in the mail, but you only need the number.
The maximum thickness for LetterPost is 2 cm, and maximum weight is 500 grams.
There are restrictions on what can be sent LetterPost, but so far this really only effects International shipping.
There is whole Board just on Shippo, which is new and still kinking a bit. On the whole it works fine, don't pay too much attention to the earliest posts when it wasn't.