No, currently you cannot add promotional material or inserts, such as a thank you card, to each order.
Although we appreciate how important is for you to have more customization options to your orders, the power of Gelato lays in our network of print partners printing locally around the world using an intelligent automated system. In order to benefit from such an automated system, one of the compromises is on more standard packaging material and process. This way we can ensure a consistent experience globally as well as a quick turnaround of your orders.