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.
In "License to Milkshake," SpongeBob discovers that his license had expired years ago and goes back to the Milkshake Academy to get a new one. In "Yours, Mine and Mine," SpongeBob is seen taking out his milkshake license briefly.
SpongeBob's milkshake license expired around the time of "Selling Out." This could be a possible explanation as to why his license expired.