Drop "billing" in invoice and use flipflop function

Change-Id: I12db0e3676191c7b406d168203ea941886c7c3f0
diff --git a/invoice.php b/invoice.php
index cbc45e6..7c7d5e2 100644
--- a/invoice.php
+++ b/invoice.php
@@ -23,10 +23,11 @@
     $invoice->setType('Invoice');
     $invoice->setReference('ICLC-10-' . $user->id);
     $invoice->setDate(date('Y-m-d', time()));
-    $invoice->lang['to'] = "billing from";
-    $invoice->lang['from'] = "billing to";
-    $invoice->setTo(['ICLC-conference / Administration', 'Leibniz Institute for the German Language', 'R5 6-13', '68181 Mannheim', 'Germany']);
-    $invoice->setFrom([($user->title? $user->title . ' ' : ''). $user->first_name . ' ' . $user->last_name, $user->organization, $user->street, $user->zip . ' ' . $user->city, $user->country]);
+    $invoice->flipflop();
+    $invoice->lang['to'] = "to";
+    $invoice->lang['from'] = "from";
+    $invoice->setFrom(['ICLC-conference / Administration', 'Leibniz Institute for the German Language', 'R5 6-13', '68181 Mannheim', 'Germany']);
+    $invoice->setTo([($user->title? $user->title . ' ' : ''). $user->first_name . ' ' . $user->last_name, $user->organization, $user->street, $user->zip . ' ' . $user->city, $user->country]);
     $invoice->setDue(date('Y-m-d', strtotime('+2 weeks')));
     // $invoice->hide_tofrom();
     /* Adding Items in table */