Inside Canada use Oversize lettermail for single thickness cds. No checking required (except by postal employees who don't read the rules).
Max thickness of 2cm and max weight of 500g. Otherwise you have to jump to the more expensive expedited parcel (same as regular parcel, but with benefits, plus an extra discount when using ebay/Shippo to buy the label).
I wrap my cd's in a thick sheet of paper and then put into a size 0 kraft bubble envelope. Then I cover it with discounted stamps (most of mine I get on eBay) for a lower shipping cost.
up to 100g -- $1.80
up to 200g -- $2.95 -- most single disk cds
up to 300g -- $4.10
up to 400g -- $4.70
up to 500g -- $5.05
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Outside Canada use either Light Packet (2cm thickness limit, not available online) or Small Packet (Air) which has no thickness limit and which has a good discount (until January) if bought through eBay/Shippo or via PayPal.
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Things may change come January 2018 as Canada Post is testing a light weight parcel alternative for Canada.
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