This is probably because you allow people to access your site from:
sportsmedicalcertificates.com
and from:
www.sportsmedicalcertificates.com
But your url that returns from paypal uses one or the other, so it might switch mid-process.
The best solution is to add an .htaccess file that forces one or the other, then make sure your confirm.php page has the correct one specified for the paypal return url to be consistent.
See:
https://stackoverflow.com/questions/4916222/htaccess-how-to-force-www-in-a-generic-way
or:
https://www.inmotionhosting.com/support/website/force-www-htaccess/