You should be able to go to the post office and get postage rates based on weight and dimensions of what you're sending. You can usually categorize shipping by Domestic (e.g. Canada to Canada), near Domestic (e.g. Canada to the US) and International (e.g. Canada to Europe).
Then when you price your product, you leave the shipping out of the equation until the order is complete, and then use an algorithm to apply the appropriate rates based on the weight of the items and the combined dimensions for shipping. After a while, you can start applying actual rates from having made shipments to make your estimates more accurate.
Also, be aware that if you are going to be doing a lot of shipping, you can probably negotiate a bulk rate with the postal service which will help bring down the cost.
The US Post Office offers a link to do this for you: link text In addition, they have some easy flat rate shipping options which are simple and work well for packages under 80 pounds - "if it fits, it ships". link text My suggestion is to either use the flat rate option and / or offer your clients the link to the Post Office site so they can see what shipping will cost.
Another option is to charge a set % of the total sale for shipping.