InventoryStores recommends the use of the UPS shipping module.  It is simple and easy to use.  It is weight based which you have for your dropship products.

However, if you still need USPS, then this guide will help you to apply your settings for your InventoryStore USPS Shipping Module.

You will need to have registered an account with USPS.  You can find the webtools for USPS at:
www.usps.com/webtools/
In the left column you will see "Access USPS Web Tools".  This is where you sign up for your account.  Most guides tell you that you must first complete a test of the connection for your account using the test server option on the USPS shipping module in your store Admin.  Some also recommend changing the sort order for the USPS from "0" to "1".

You will need to have registered an account with USPS.  You can find the webtools for USPS at:
www.usps.com/webtools/
In the left column you will see "Access USPS Web Tools".  This is where you sign up for your account.  Most guides tell you that you must first complete a test of the connection for your account using the test server option on the USPS shipping module in your store Admin.  Some also recommend changing the sort order for the USPS from "0" to "1".

While there is still the server setting for a test in the module options, the test no longer appears needed for the USPS module.  You just have to sign up at www.uspswebtools.com. You will receive your webtool username/password via email. Then call the following number and mention you're using osCommerce:
The ICCC is staffed from 7:00AM to 11:00PM Eastern Time.
Telephone: 1-800-344-7779
E-mail:

They will activate your account over the phone. Remember to make sure your details are added to the admin of your store, and change the setting to the production server.

Some possible errors that you might have from the USPS Shipping Module are:
Error 1:
"United States Postal Service
An error occurred with the USPS shipping calculations.
If you prefer to use USPS as your shipping method, please contact the store owner. "
This error often occurs if you have not contacted the ICCC to activate your InventoryStore USPS shipping module.  (See above for the contact details.)
 
Error 2:
"United States Postal Service
This Information has not been included in this Test Server."
IThis error means that the User ID that USPS has for you is not matching what you are sending them. It could be a type in your Admin or on your account with USPS.  You can continue to check your store settings and code, if you are unsure of where or how your InventoryStore site was installed.  If Inventory Source installed the site, then the loading was done correctly for your version.  You might find a quicker resolution by contacting USPS directly and verify the details in your USPS account and your store Admin settings. (See above for the contact details.)