Automatic Flat Rate Shipping for Assigned Box Size?

[ftbnms]

I feel like I'm missing something in regards to shipping rates for our online store. For each item listed online, I have assigned a specific sized flat rate box. Under our fulfillment settings, I have a shipping profile set up for the U.S with different rates for each sized flat rate box. However, when a customer is checking out in the online store, despite the assigned shipping box, the customer can choose the shipping rate. 

I'm wondering what is the point of assigning a shipping box to each item if the customer can just choose the box anyways? Is there a step that I am missing? Any help or suggestions would be greatly appreciated.

[balmonte]

Hi - could be wrong but it looks like the box feature is used for real time shipping rates, and not for flat rates that you have set according to the description in the item creation screen. It indicates "Add the box that this item will ship in to get real time shipping rates calculated at checkout." 

1. Does that happen even if you add one item to your cart?
2. What happens if you add multiple items that contain different box sizes?

Just curious what the experience is in those instances and might shed some more light on what you are trying to achieve.