Setting up your Ecommerce Plus template to automatically use USPS shipping is pretty simple as the majority of the the integration has been done for you. Just follow the steps below.

US and overseas shipping costs are automatically calculated according to product weight and destination. To activate USPS shipping method you need to register for a username and password. The process is quite simple and the registration form is available at . . .
http://www.uspsprioritymail.com/et_regcert.html
Make sure you follow the link for the API password request, not the "Shipping Assistant".

You will receive a long email from USPS with information regarding testing the system before going live on the production server. We have already done this step for you so that can be ignored. All that is needed is to follow the instructions for going live on the production server.

The final step for using USPS shipping is to enter your ZIP code in the admin main panel along with your USPS username and password. These are used in determining shipping costs within the USA.

Once you have selected USPS shipping in the admin main page then you should see a link "Edit UPS Shipping Methods" on your admin home page and that is where you can select the shipping methods that your package shipment can use.

There's no more to it than that really, and as all the integration has already been provided in the software you should be up and running with USPS.

All that is left to do is to add the weight of each product as they are added through the product admin section. The shipping cost will then be calculated on checkout depending on the origin, destination and weight of the order.

IMPORTANT NOTE: After you apply to the USPS, you will receive an email from them with your username and password. In this email will be a couple of lines about how to switch your profile to allow you access to the production server. Until you follow this step you will receive an error in checkout about not being authorized on the server. Remember to mention to USPS that the set up is "pre-tested" and you are using Ecommerce Templates.