Heh, your airline ID and €4 will get you a shot of espresso at BRU...train discounts, not so much.

Two possible ways to do this more cheaply. Walk up fares are slightly cheaper for TGV trains with a connection at Lille Europe than direct Thalys trains, and it takes about 2 hours instead of 1:20. A far better deal is to buy your ticket online ahead of time (tgv-europe.com) - with enough notice or at off-peak times you can get no-flex fares on Thalys for as little as €40.

FYI, most direct trains to Paris depart from Brussels-Midi. It's about a 20 minute ride from the airport.