firstname.lastname@example.org is our PayPal business Member ID and the one we populated in the Checkout wizard.
When we navigate through the website, we are taken to the PayPal Home Page after selecting the Submit Order button on the Confirm.php. Is this the button you're selecting?
I confirmed this email address on the Website Payment Standard printout that I took during my PayPal account setup. Very confused why yours works and mine doesn't. Is there a way you can use mine to test with?