Originally Posted by
caliform
It seems Delta doesn't sell the Premium Eco product through their website. Just Main Cabin or Business, which makes sense as it's not really the same as the Comfort+ product.
Delta absolutely sells Premium Economy on partner carriers that offer it (AF, AZ, VA, VS). It's confusing because of the mess of a rollout of the "Comfort+" product, which also books into W (though using a different mechanism).
If you search for regular coach fares, Premium Economy should be shown as an option on routes that offer it. If you do an advanced search for "W (Comfort+) or higher", but search for a route with partner Premium Economy, you will also get Premium Economy in the search results. These will always book into DL W, regardless of what the corresponding partner fare class would be.
One complication is that AF often routes onto non-SkyTeam partners (eg: all CDG-HEL and some CDG-VIE). DL will not offer these routings even though AF will.