It's called a commercial invoice and every major shipping company, including Canada Post has their own.
You basically fill it out with your shipping and contact info, CGC's shipping and contact info, and a description of what is inside.
The form is attached to your package. You keep one copy, the shipping company will keep one copy and CGC receives and returns a copy of the same invoice when your books are sent back to you.
I've literally shipped 100's of packages to CGC and never had a problem.
The odd time that I have forgotten to add the commercial invoice I was simply charged a small ($15 or so) customs fees.
Hope this helps.
Roy, If you can point me in the direction of Canada Post's Commercial Invoice, I'd sure appreciate it. I've looked on their website and I can't find one anywhere. Today I visited two postal outlets, one run in a Shoppers Drug Mart store (with knowledgable staff, I might add) and also the main Canada Post depot in my city. Clerks in both locations had no idea, and both claimed Canada Post had no such forms. If you've got some, please advise me where I might get one. Many thanks. -- Chris