It is a purple license card, with bold letters at the top that read out "Milkshake License." Below that, there is text that reads "Class D" and 3 check boxes labeled "Chocolate," "Vanilla," and "Strawberry." At the bottom, SpongeBob's name is signed in cursive writing.
SpongeBob's milkshake license expired around the time of "Selling Out." This could be a possible explanation as to why his license expired.