Have a lok at the README.txt that comes with the Commerce USPS module, here is the content of it:
About this module
Commerce USPS module provides shipping calculations from the USPS
This module depends on the Commerce module
In addition, the following modules are required:
Install and enable the module and all dependencies (be sure to use the latest versions of everything). Add dimensions and weight fields
(new field types via the Commerce Physical module) to all shippable
product types. Populate dimensions and weight fields for all products.
Configure the "Shipping service" checkout pane so that it is on the "Shipping" page. The "Shipping service" checkout pane MUST be on a
later page than the "Shipping information" pane.
Configure the USPS settings (admin/commerce/config/shipping/methods/usps/edit). You'll need to
create USPS WebTools account and obtain a username via
Eventually, all of these limitations may be addressed. For now, be
Single "Ship from" address for all products.
Doesn't ensure product dimensions are less than default package size dimensions. In other words, if you have a product that is 1x1x20
(volume=20) and your default package size is 5x5x5 (volume=125), even
though the product won't physically fit in the box, these values will
be used to calculate the shipping estimate.
Doesn't play Tetris. For example, if you have an order with 14 products with a combined volume of 50 and your default package size
has a volume of 60, the shipping estimate will be for a single box
regardless of if due to the packaging shape they don't actually fit in
Doesn't limit the weight of packages. If you're trying to ship a box full of lead that weighs 600lbs, this module will let you
(instead of breaking the order into more packages).
Doesn't account for packing material. If you need to account for packing material, then you may want to adjust product dimensions
Calculating estimated shipping costs is a tricky business, and it can
get really complicated really quickly. Knowing this, we purposely
designed this module with simplicity in mind. Here's how it works:
Every order must contain at least one package.
The number of packages is determined by calculating the total volume of all products in the order, dividing by the volume of the
default package size, and rounding up.
The weight of each package is determined by dividing the total weight of all products in the order by the number of packages.
If you need custom functionality, you have several options:
Determine if it is something that can be generalized to suit a number of users and submit it via the issue queue as a suggestion for
inclusion in this module.
Hire one of the maintainers to create a custom module that interfaces with Commerce UPS to add your custom functionality.
Break open a text editor and start coding your own custom module.
Apart from that, the module's project page contains this extra info:
Please refer to the README.txt file for additional information about how this module works and how to set it up.
Install and enable the module and all dependencies.
Add dimensions and weight fields (new field types via the Commerce Physical module) to all shippable product types.
Populate dimensions and weight fields for all products.
Configure the USPS settings (admin/commerce/config/shipping/methods/usps/edit). You'll need to create USPS Web Tools account and obtain a username https://secure.shippingapis.com/registration/ and call USPS to have the account moved into production (tell them you are using 3rd party software).