OK - thanks. I get the difference now.
Have just spent some time on phone with PayPal. I am now configured to use Payflow Pro as the gateway. PayPal tells me that as far as they can see everything is properly set up within the merchant account configuration. I have removed the existing API credentials and reinserted newly requested API credentials from PayPal, into the Gateway settings in PS4. I am not in test mode, and am not set to use the sandbox. When I submit the order, I get
Your transaction could not be completed. Either your credit card information or billing address has problems and cannot be processed
Please return to checkout and try again."
PayPal tells me that the order never reached them. Can you help me diagnose this problem, please?