Any flight outside of the 50 states will be branded as Envoy. I believe this is because there are criteria that has to be met to be considered "first class" when flying internationally like 180 degree seats and cerain levels of service. Since the short haul international flights don't offer that they cannot be called "first class." Unfortunately these fights also don't have all the things long haul Envoy has like AVOD. (even if on the 757 or 767 they won't be activated.)

All 757's with flight numbers above 700 (US "east" metal), and 767's are outfitted with Envoy seats. But like you thought the A32x and 737's have the standard domestic first seats.

If it doesn't matter to you where your CUN flight originates I would recommend PHL as it's over the 3 1/2 hour mark that will get you a meal in first. The CLT flight is a little short of that mark. I've heard rumors that they will start adding meals to flights that are a little short of the mark but I don't think that starts until they bring glassware back in April of this year.