Stop at your local PO and ask for parcel rate flyers. Get any rate flyers they have handy, but be aware that there will probably be a rate rise in January.
Very generally, the cheapest rate is Letter. These must be 2cm or less thick and weigh 500grams or less.
And all the rates are based in metric. And have been since 1974. If you use any other system you will make errors.
Outside of Canada there is also a rate called Light Packet which also has dimensional pricing of 2cm and 500 gm or less.
Neither of these has insurance or tracking /delivery confirmation.
Within Canada if your item is either over 500 grams or thicker than 2cm you will be using Regular Parcel. There are two rates, National and Regional. I just sent a 3cm /128 gram package to Montreal (from Ottawa) for $8.80ish. If it had gone to Alberta, that would have been about $12. However, this service is both insured up to $100 and Delivery Confirmed.
Outside Canada, the next 2cm/500gm plus service is Small Packet Air. Don't EVER EVER ship overseas by Surface. Just don't. And don't allow your customer to demand it.
With the exception of Letter, if you use Calculated Shipping on your listings, your customer will see the cost of shipping to his postalcode when he checks. You can see this yourself on a listing that uses Calculated Shipping. I usually enter Belgium for overseas and 90210 for USA to see what my customers will see.
