To troubleshoot checkout problems in power store, I put the following on the checkout_success page:
<?php
if(!session_id()) session_start();
echo("Paypal Full Response :<br />");
echo("<code>".isset($_SESSION["PP_DirectPayment"])?$_SESSION["PP_DirectPayment"]:""."</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("UPS Full Response :<br />");
echo("<code>".isset($_SESSION["WA_Store_Cart_UPS_FullResponse"])?$_SESSION["WA_Store_Cart_UPS_FullResponse"]:""."</code><br /><br />");
echo("FedEx Full Response :<br />");
echo("<code>".isset($_SESSION["WA_Store_Cart_FedEx_FullResponse"])?$_SESSION["WA_Store_Cart_FedEx_FullResponse"]:""."</code><br /><br />");
?>
This tells me the response from payapl, USPS, UPS and FedEx so I can see where the failure is coming from.
Glad to hear you sorted it out.