Hello, DB. As for shipping light packets, that what I do almost exclusively.
If insurance isn't required, and shipping time isn't an issue, Small Packet (Surface) to the United States is the cheapest. The cheapest package starts at $3.35. If you want to send it air, the cheapest package starts at $5.05.
This is up to 250g. From 250g-500g, the costs go up a bit, but still quite reasonable ($5.10 and $6.70)...
The full breakdown for sending to the united states is here:
http://www.canadapost.ca/business/offerings/small_packets/us/rates-e.asp
Now, for sending to Canada, as far as I know, there isn't much option with Canada Post. Just make sure you have a VentureOne card (you get a 5% discount). It gets there quite quick, with up to $100 CAD insurance for free. For me, living in alberta, I can ship to BC for about $7.00, Alberta for $6.00, Ontario (etc) for $8.50, and the maritimes for $9.00. These are estimates -- it depends on the weight/cube, etc.
Hope this helps!
John