As pointed out by @mkennedy, Canada Post has a service called International Reply Coupon, which, as per their web-site, appears to be a one-size-fits-all that's valid in 191 countries (except for Taiwan), bearing a cost of 6,00 CAD. https://www.canadapost.ca/cpo/mc/business/productsservices/atoz/irc.jsf
However, the problem comes from the fact that, as its name suggests, it's not an actual stamp, but rather a coupon, which must be redeemed for a local stamp at a local post office in the country from which the reply mailing is to be made. (The usage procedure is actually even documented as such right on their web-site.)
The above would go in contradiction with the requirement of having a "pre-paid envelope" -- after all, instead of attaching said coupon, it might as well be easier to provide a two 1 USD bills (see cost below), which, after all, can surely be be exchanged for adequate postage with more ease than having to deal with some sort of coupon most people have never heard of, and which appears to specifically require a visit to the Post Office.
As per Can I combine two French stamps?, it generally costs only a couple of times more to post letters internationally than domestically. As of 2015-05-09, one domestic first-class USPS stamp with a denomination value of FOREVER costs 0,49 USD (the FOREVER stamps will correspondingly and automatically increase in value if the price to post goes up), and such one stamp is sufficient for posting any small letter from any state to any other state. As per http://ircalc.usps.com/MailServices.aspx?country=10054&m=1&o=2.0000, it currently costs 1,15 USD to mail a letter at or below 57 grams (2 ounces) from US to Canada, thus three such stamps at 1,47 USD should be sufficient to post internationally.
As such, the easiest solution would be to buy and affix three FOREVER stamps to the reply envelope, either within Canada, or before making a trip there.
Additionally, it seems like USPS also has the specifically designated GLOBAL FOREVER USPS stamps, too, which are currently valued at exactly 1,15 USD, and will always be sufficient for international postage.