Before the site is deployed with it's final URL, my webhost has given me a URL with a subfolder to use during development. So my site's url right now is something like http://184.108.40.206/wordpress/
After payment is submitted, the Paypal Add-On plugin is returning a url like http://220.127.116.11/page-slug/?transaction_subject=5|c7d373c65884b04a3d9892a.... and omitting the all important subfolder!
I ran into this issue with the Action field in the form generated by main Gravity Forms plugin, but I was able to make a simple temporary edit to the form_display.php file to add the full url to the Action field.
Where can I modify the PayPal Add-On plugin to send PayPal the correct "merchant return link" for my site?