I've shipped a few coins in my day. For cheap ones (under $5) I just put them in a coin flip, then in a small zip-lock bag (found in the craft section at the dollar store) and then mailed them in a plain white envelope with a stamp.
Anything a bit more valuable you might want to mail in a bubble mailer but it'll still be thin/light enough to send Oversize Lettermail for a few bucks in stamps.
No tracking on Lettermail so there's a risk of your buyer filing an "Item Not Received" case, but that's just the risks we have to take in Canada to remain competitive.
All of the above is assuming it's just going within Canada. If it's leaving the country it needs to go as a parcel with a customs declaration and I would at least put it in a coin flip --> zip-lock baggie --> bubble mailer.
