UPDATE!
I changed the trigger in the UE Server Behavior to "any form post" and the email gets sent when clicking the Continue button on the checkout.php page, but it sends the email again when clicking the Complete Order button. I can also get the email to send by changing the trigger in the UE Server Behavior to "eCart: my eCart Object is empty" then hit the back button after completing the order. BUT, it still will not work with the trigger set to "Button: Payment_submit pressed", which is what I need.