the issue is the shipping failure, shipping is calculated at different times in the process using express or standard.
add the following code to the checkout failure page to write the full request and full response from fed ex to the screen for debugging:
echo "<p>FedEX: <br />";
echo "Full Request: ".(isset($_SESSION["WA_Store_Cart_FedEx_FullRequest"])?htmlentities($_SESSION["WA_Store_Cart_FedEx_FullRequest"]):"n/a")."<br />";
echo "Full Response: ".(isset($_SESSION["WA_Store_Cart_FedEx_FullResponse"])?htmlentities($_SESSION["WA_Store_Cart_FedEx_FullResponse"]):"n/a")."<br /></p>";
from the poser store documentation for setting the IPN page:
If you are using PayPal Standard as your payment gateway, you will be provided with an IPN page URL. This URL will need to be added to your PayPal Standard account details on the PayPal website.
1. Copy the IPN page URL that appears on the Gateway Settings tab of the Settings page in your Store Manager.
2. Log into the PayPal website at paypal.com.
3. Choose the Profile link from the sub main navigation.
4. Under Selling Preferences, choose the Instant Payment Notification Preferences option.
5. Select Choose IPN Settings.
6. In the Notification field, paste the IPN page URL that is provided in the Store Manager.
7. Select the "Receive IPN messages" checkbox.
8. Click Save.