I'd actually rather put "Past Roundtrip Deal" above the deal info just because of how big the word "Roundtrip" is. We have to keep design in mind... and if we just put RT that would likely lead to confusion as well.
"NYC to Paris and back: $260" is only one fewer letter but I feel it reads more clearly and you don't need to include the "and back" in other lines of the message to get the point across you're comparing RT to RT.