Thank you for helping me with this! (And on a Saturday ta-boot!)
That helped narrow it down. It’s working now. HURRAY!
(On the default mode – it worked. So, out of curiosity, I deactivated, uninstalled, and reinstalled BP security. Then placed the custom code back in and clicked the automagic button and what not.) Transaction went through on live paypal, with no 403 errors.
Thank you! Thank you!!!!!!!!!! 😀