If you've negotiated a discounted rate with USPS, then you can enter your USPS credentials in plug&paid when you activate the USPS service in your Settings so that the discounted rate appears to your buyers in the shopping cart.
Register for a USPS User ID
Register for a USPS Web Tools user ID by filling out the USPS Web Tools registration form.
When you have completed the signup process, USPS will mail you a user ID. Make a note of this, as you will need your USPS user ID to sync your USPS account with plug&paid
Your USPS Web Tools user ID is a combination of letters and numbers, for example
123ABCDE1234, that is automatically generated by USPS.
The user ID will give you access to package tracking, rate, and service delivery standards APIs with USPS when you register.