When you attempt to go back to your site from PayPal what page are you getting to in the end and is there any error message displayed? It sounds like your checkout or shipping server behavior are failing. In order to determine what the problem is you will need to add in the debug info to the page you are getting to in the end. You should add in the USPS full request and response as well as the PayPal ECO Process Transaction Full Request and Full Response. Here is the code that you can use for these values:
<?php
echo("USPS Full Request :<br />");
echo("<code>".isset($_SESSION["WA_Store_Cart_USPS_FullRequest"])?$_SESSION["WA_Store_Cart_USPS_FullRequest"]:""."</code><br /><br />");
echo("USPS Full Response :<br />");
echo("<code>".isset($_SESSION["WA_Store_Cart_USPS_FullResponse"])?$_SESSION["WA_Store_Cart_USPS_FullResponse"]:""."</code><br /><br />");
echo("Preocess request: ".isset($_SESSION['"ECO_Process_Request'])?$_SESSION['ECO_Process_Request']:""."<br /><br />");
echo("Process Result: ".isset($_SESSION['ECO_Process_Result'])?$_SESSION['ECO_Process_Result']:""."<br /><br />");
?>