Originally Posted by
ijgordon
You can usually book "Premium Economy" via Amex, which for a domestic flight will be Comfort+. On an international flight, PE is PE. It would be interesting if the purchase flow had the option to upgrade/upfare to C+ (but not PE). But doing so post-ticketing on Delta.com should certainly be a workable fallback.
There is an option to upgrade to comfort plus on international flights in the booking flow on Amex as well. I don't know the OPs dates to replicate it exactly but I selected random dates in August and chose DL flights and am offered the following:
There are additional options of Premium refundable, Delta One, and Delta One refundable as I scroll down.
Interesting to note that Amex considers Comfortable plus to be Premium Economy cabin on international flights as well.