I'm having trouble setting up the Square Payment Gateway. I set up the App ID, Access Token and Location ID, but the iframe for the square payment form is blocked.
I was able to figure out part of the problem. Adding *.squareupsandbox.com to /config/secureheaders.php allows me to successfully display the form. After I enter a test credit card, the invoice is successfully marked as paid but, I'm receiving a 500 server error at https://mydomain.com/payments/square/checkout