The Secret Shipment is a box similar to the Lockboxes that is delivered directly to your Home, when opening it you will receive a reward. After opening it you will receive another after 30 minutes, the time until the next shipment can be seen by accessing the Town. You will receive a notification when you receive a shipment if you are not at home.
They come in 3 different colors being Green, Blue and Red, there is no Silver or Gold.
The contents of the secret shipment most of time are items related to Converters.
Note: According to official information, the color of the secret shipment changes and the contents of the chests improve according to the amount of furniture in your home.
Check out the list of rewards in each box that has been confirmed in the different types of boxes.