The Admin could find user email useful, and it is at least as meaningful as user_id.
modules/Storez/mail/receipts.php around line 113:
	if ($orderInfo['userType'] == 'user') {
// layingback - add email - start
		$result = $db->sql_query("select user_id, username, email from ".$prefix."_cart_users
		WHERE user_id='$orderInfo[userID]'");
		 list($user_id, $username, $email) = $db->sql_fetchrow($result);
		 $mailMessage .= _STOREZ_RECEIPTS_USERTYPE.": User\n";
		 $mailMessage .= _STOREZ_RECEIPTS_USERNAME.": $username [$user_id] [$email]\n\n";
// layingback - add email - end
Initiated by Bugger (Original Entry) - Comments Here Please.