You can add multiple designs to you cart. Simply design the first item and add it to your cart. Once the item is in your cart, hit the "Keep Shopping" button to start designing a second item. Keep repeating this process until you've designed all of your different items.
Where is the keep shopping button?
The Keep Shopping button is found on the Review Your Cart cart page, which is the first step in your Checkout Process.
View of the Keep Shopping on a desktop:
View of the Keep Shopping on mobile: