"You pay us to be advertising for startups"... Im sure some will signup out of love for YC but have you thought of instead paying people to receive the T-shirts with the hope that they will wear them in public?
HN Tees: Provides the service of processing and distribution for the startup.
HN Tee's member: Pays $15 for a monthly HN Tee because its a high quality t-shirt designed by an awesome Startup, they like and wear a lot of T-shirts, and follow and support YC Startups.