Hi!
The link above has lots more info.
Because you've booked with DLP direct you could ring them and add disney express for both your journeys to your existing booking, or you could book just the return journey at the hotel while you are there (see post 29 in the link for a pic of the sign in the hotel).
But disney express for just the return journey means you leave all your cases at the left luggage in your hotel, you'll get one slip for each case. They will then take the cases to the station for you and you go to the luggage part of the station with your slip to pick them up in time for your train. Very easy to use as long as your train times are right. If you're on the disney direct eurostar its a huge advantage as it takes a lot of the stress out and you don't have to go from the park back to the hotel for luggage and then back to the station.