If that coin were sent to the UK (for example) then many postal workers would insist that it be sent Small Packet Canadian International, because the seller is not sending paper / documents. The cost for that is $9.76 Canadian at the post office. Some postal workers might err and charge the 100 gram letter rate which is $5.90 Canadian, once again because the contents of the envelope are not paper.
I don't mean to be critical, but I've been caught with this at Canada Post before. A coin might fit in an envelope that fits through the narrow slot and it might be light (up to 30 grams or up to 50 grams), but if it's not paper, the Small Packet rate is supposed to apply.
And this doesn't include any applicable goods and services tax (GST), or consider the costs of paypal fees or other expenses of the sale.