Just tested it, and it seemed to work fine...however i changed your if statement a bit:
if ($payment_status == 'Completed' && $receiver_email == 'zoob@pop3.com' && $payment_gross=='0.02')
{
echo "$display";
}
else
{
echo "$error";
}
...but the one you made should be ok too.
If i understand correctly you wish to validate a specific set of requirements from a given input form that is sent to the page.
And if the requirements match your criteria, you wish to display a button that will allow the sending of a mail to the ëmäil that went along with the post, else just display an error page.
If the above assumption is correct, then remember to send along the information that you need to the next page, eg: make 3 hidden input fields and throw along the paystat, patgross, and ëmäil.
If this is something that a user has to do , you might considered jumping the above click button step, as in: send infomation, validate silently and just send the mail.
hth