Link directly to register form?

I have got some buy buttons linked directly to the registration form as I would like it to be a neater step by step process. I got this url from the page when I was viewing it logged into another account

http://www.cosmopolitanwellness.com/register-2/?action=registeruser&subscription=1

It works perfectly to show the form and then adds the user. However I then get this error

Warning: Cannot modify header information - headers already sent by (output started at /home/jessmembers/cosmopolitanwellness.com/wp-content/themes/twentytwelvechild/header.php:13) in /home/jessmembers/cosmopolitanwellness.com/wp-includes/pluggable.php on line 680

Warning: Cannot modify header information - headers already sent by (output started at /home/jessmembers/cosmopolitanwellness.com/wp-content/themes/twentytwelvechild/header.php:13) in /home/jessmembers/cosmopolitanwellness.com/wp-includes/pluggable.php on line 681

Warning: Cannot modify header information - headers already sent by (output started at /home/jessmembers/cosmopolitanwellness.com/wp-content/themes/twentytwelvechild/header.php:13) in /home/jessmembers/cosmopolitanwellness.com/wp-includes/pluggable.php on line 682

Is this something to do with my linking directly to register form or is it something else?