Help with Canadian returns postage

[Morphindel]

So the Canadian post office website is kind of confusing me, so I was hoping there was some of you lovely Canucks in here can help clarify for me. I had a quick look at the sticky thread but it hasn't been updated since 2009 so i don't know how much is still valid.

I'm thinking of writing to someone shooting VV in Canada, and I'm wondering how many stamps I should send for returns postage to the UK. I usually send an A4 (i believe it is C4) hard backed envelope, and I have 8 international Canadian stamps each valued at $1.85 each. Now i guess this would count as oversized, and probably comes to about 120g, but the post office website prices it as about $5.80 for oversized international post-- so does that mean I should put 4 of these international stamps on? I'm just trying to get it clear in my head before I go throwing away my precious stock of Canadian stamps. one book I had to wait about 3 months for it to be sent from Canada, and the other book I bought whilst i was over there last year.

Anyone here able to just confirm that for me?

thanks for the help