I took a quick look on this one.

First of all, while it is indeed labeled on the cover page as a "flexible" fare and "free cancellation", if you read the detailed fare conditions, they are those for a "lowest" business fare: not refundable, $300/400 penalties for changes, etc. In fact, while I didn't check every word, in crucial elements it is the same language as the conditions on ExpertFlyer for a "P7SUNCT" fare, the lowest available restricted, not flexible fare.

Second, on top of those conditions, Cheapoair says that making changes will require paying the AC fare difference, AC penalty, **plus unspecified Cheapoair service fee**. That sets off more warning bells in my head.

As to price, it does price out a bit more cheaply (~10%) than a P7SUNCT or P7LCNT fare on ITA matrix for the same dates, but not much (after FX); and of course a lot cheaper than a true flexible and free cancellation fare. The fare rules language in the detailed fare conditions has been partially edited, but has left in language that suggests it is a "contracted fare" with Air Canada, which is something some of my clients have with AC as well (with varying levels of discount), but with strict requirements to be used for their employees/contractors only, not to be (re)sold to the general public. So I may be wrong, but I suspect Air Canada might be quite unhappy once it finds them being sold in this manner (it of course depends on the language of the contract Air Canada signed with whoever is providing these tickets to cheapoair, which we have no way of knowing).

All this combined would make me hesitate to buy from them.