I'm not sure I understand what you want, but I believe it's one of these:

1) Emailing works, but you want to change the recipient depending on a checkbox. Add:

$recipient = 'test@test.edu' if ($FORM{'ckusno'});
[download]

2) Emailing works, but you want to send the email to two recipients depending on a checkbox. Put the mail stuff in a subroutine and call it twice:

$recipient2 = 'test@test.edu';

sub mail_it {
   my ($to) = @_;

   open (MAIL, "|$mailprog $to") || die "Can't open $mailprog!\n";
   print MAIL "Reply-to: $FORM{'espr2'} $FORM{'estb'} $FORM{'espr'}\n"
+;
   print MAIL "MIME-Version: 1.0\n";
   print MAIL "To: $to\n";
   ...
   print MAIL "</body></html>";
   close (MAIL);
}

mail_it($receipient);
mail_it($receipient2) if ($FORM{'ckusno'});
[download]

In the future, please specify what it does wrong, not just what you want it to do.