Long running deal that has never been advertised, I think they purposely keep it secret? Every week they have a different drink or sometimes 2 different drinks that you can purchase. You get a large for the price of a mini ($6.50 instead of $8.90 or $7.50 instead of $9.50). Just have to go in store and ask for what the tank of the day is (if you don't ask they will charge you full price).
Edit: Apparently it's advertised every Monday on their facebook page.