According to this Steam Inventory thingy(the link will show a different inventory for you, obviously), I have a 33% off coupon for Valve titles:
33% off any one title on Steam listed with Valve as the publisher.
However, Half-Life: Source does not appear in the List of applicable games for that coupon. Even though the Game Details say
Title: Half-Life: Source
Genre: Action
Developer: Valve
Publisher: Valve
Release Date: 1 Jun 2004
So how can I use this coupon to save the incredible amount of 1,65 EUR?