The ac.com website is often, um, not very good as a search engine (though sometimes paradoxically it offers better than otherwise available fares).

However, when I have found lower AC-only fares on ITA Matrix (the underlying, more detailed search engine under the hood of Google flights), I've so far always been successful in getting AC to ticket them over the phone, equipped with the detailed fare information displayed on the final ITA Matrix screen. Unlike some of the discount fare sites, ITA Matrix doesn't get a cut so doesn't play shenanigans. (It also doesn't offer true consolidator fares, ones that others sometimes do, though with less weird inconsitency than this particular instance.)

If it is a constructed fare combining several carriers on one ticket, then you need a friendly bricks-n-mortar travel agent to issue it for you. That's the main reason I have one, one whom I generally give precise instructions to based on looking things up myself on ITA Matrix and expertflyer. He charges a ~$30/ticket fee but quite worth it when it's too complicated to self-issue on an airline website -- or when I need to make changes while travelling and have poor connectivity.

I may have come down too hard on cheapoair.com in my response. They may well be offering a discounted consolidator fare from a block of tickets they have acquired from AC and are reselling quite legitimately. It's just that their general reputation and the inconsistencies between what's on their cover page description of the fare, and in the detailed fare rules, makes me suspicious.

Good luck!