Keesa,
Don't get discouraged! All of us know what you're going through :) I host the Canadian shipping thread that Eightmile mentioned above. As you go along with your shop, you can post any and all your questions there.
For the type of items you'll be shipping (Oversize Lettermail within Canada, Light Packet for USA/Int'l), you can buy Light Packet postage on Paypal but not Oversize Lettermail. There's no difference in price for Light Packet -- you'll pay the same on Paypal as you would at the post office. On Paypal, the discount you get only applies to services that have tracking (Regular/Expedited Domestic Parcel, Tracked Packet, Xpresspost).
For estimating parcel rates within Canada, take a few postal codes from different cities and plug your parcel weight/dimensions into the Find A Rate function on the CP website. For example, if you live in Toronto, use a postal code in Toronto, Winnipeg, Calgary, Vancouver, Montreal and Halifax (you find them with a google search). Play around with it for an hour or so, using different weights/dimensions. You'll start to see the pattern of rates which you can use to start pricing the shipping for your items.