The checkbox for "Same as Registrant" isn't working in any 2.7 version and I think is causing issues with authorize.net (names not transferring). In what file can I delete it until this is fixed?
What I am saying applies to version 2.6.9 (probably similar for 2.7)
The Text "Same as Registrant" is in the language files (website root > language > en-GB (or your language) > en-GB.com_dtregister.ini)
To remove the "Same as Registrant" line altogether, try the file "dtregister.php" located in (website root > components > com_dtregister). Look around line 17,384 or try searching for this tag "DT_SAME_BILLING_INFO" -- then remove or comment out (<!-- -->) this entire line "<td><?php echo JText::_( 'DT_SAME_BILLING_INFO' );?></td>"