The extension supports defining which type of USPS flat rate box each product ships in. This can be in addition to the individual or multiple shipping boxes you may have defined in the previous step.
The extension will request a rate for USPS flat rate boxes when all products in the cart are assigned to a USPS flat rate box. If there’s a product in the cart which isn’t assigned to a flat rate box then rates will not be requested for flat rate boxes. Only standard USPS services will be returned.
If products are assigned to different sized USPS flat rate boxes then the largest box will be shown as the choice to the customer, the price will be for the largest box plus any other smaller boxes required by other products. For example:
Product A is assigned to a small box which is $5.80
Product B is assigned to a large box which is $16.85.
The customer will see “Large Flat Rate Box” @ $22.65 ($5.80 + $16.85)
The fields in this configuration area have the same functionality as the ones under “Individual Shipping Boxes”. The length, width and height are irrelevant in this section as the flat rate boxes are of a fixed size. The fields will always be greyed out. You can either use a predefined USPS Box, or one you’ve created under the catalog->manage shipping boxes menu. You will only see boxes which aren’t of USPS Box Type custom.
Boxes are packed in the same manner as the Exact Packing algorithm. For a detailed explanation please see the article: How does Dimensional Shipping work out which individual box items fit into?