Fortunately no, this would be slow and unnecessary. Postie intelligently caches API requests based on the contents of your cart, and address information. If nothing apart your cart or destination address changes, then neither do the shipping calculations.
Postie creates only one box to fit all line items into, using a simple box packing algorithm:
length- maximum length of all items
width- maximum width of all items
height- total height of all items
Far from perfect, this solution caters for most general cases. More intelligent and custom boxing packing algorithm solutions will be introduced into Postie soon.
Available for Craft 3. Get it from the plugin store.