To debug this issue, pleas open the WA_eCart_USPS_PHP.php file which is located in your WA_eCart folder, and look for this line of code:
$WA_USPS_Response = curl_exec ($WA_USPS_Curl);
and replace it with:
$WA_USPS_Response = curl_exec ($WA_USPS_Curl);
print("<pre>");
print_r($WA_USPS_Response);
die();
save the page and try to check out again. On the confirm page, you will get a text output which will include the response from USPS. Please copy and past the output to a file and attach the file to this thread.