Make sure you can authenticate who you deal with and make sure you have a good level of trust. There are plenty of people out there that will take your BTC and not send the CAD. But if you deal with someone who has a long public record of honesty you should be ok. The WOT on bitcoin-otc.com is a good way to evaluate and authenticate who you deal with. You don't need to enter the order on that site but you can use the WOT to evaluate and then rate each other when done.
If you're not familiar with IRC you can use a web client and just follow the steps in the help guide.
Wait a minute [WTB] BTC means you want to buy BTC. Shouldn't you have,
[WTS] BTC for CAD in mail
[SELL] BTC for CAD in mail
and give some details of how they should mail it. Wrapped, brown envelope with tracking? Or whatever, I don't know how people usually send cash in mail. I had a pot grower friend a decade ago who apparently regularly sent $10,000 in the mail. I don't have the stomach for that.