You have have an account with CP's Online Business Centre to get these free envelopes (they're not bags, they are 12" x 16" poly envelopes like you see given out by Purolator, FedEx, etc. but they're big enough to be used as bags if you really need stinky bags with no handles that badly). They're very good quality and keep contents almost better sealed than Tupperware, and have the eBay Power Seller logo on one side. Depending on what you ship you could give up buying bubble envelopes and get bubble wrap to use with these envelopes. You will also need a credit card to order these, even if your order totals $0.00
Go to canadapost.ca, click on "Go to Business" then on "On-line Business Centre". If you don't have an account you can apply for one from here.
Once logged into the OBC, click on "Order Products and Supplies" and do a Quick Search for "ebay". That should bring up the envelopes. You can also get a pack of 100 adhesive pouches that works well with these envelopes, and are also free, even though it says they cost $7.22 -- when you put them in your shopping cart the cost magically goes to $0.00 - too bad it doesn't work on everything in their store. Many of the labels -- Air Mail, Small Packet, Expedited, Fragile -- are also free.
Cheers,
Ernie.