I wanted this payment option, and I couldn't find it. A forum search revealed a guy who apparently got it "mostly" working (but not editable through Admin) last March. I haven't heard back from him, so I thought I'd try importing it from the OsC contribution myself.
It was pretty easy to do, I'm just posting this here to maybe save some other Canadians some time having to import it from OsC themselves.

I've tested it on Zen 1.3.6 and it works for me, though I would appreciate some testing/input from others! ^_^

Download .zip here: http://www.kittenfoot.com/downloads/...r_Zen_v1.0.zip

It's based on the Money Order module, so it doesn't actually send customers to Certapay.com, but it emails them instructions on how to pay you. You can edit your email address and other info through the Admin.