Display net sum on invoice

Change-Id: I90982ec65fd4aabb3f71bc0331741750edc8eca5
diff --git a/invoice.php b/invoice.php
index 5d888af..a3bdf08 100644
--- a/invoice.php
+++ b/invoice.php
@@ -83,10 +83,10 @@
     }
 
     /* Add totals */
-    $invoice->addTotal('Gross', $user->total_due);
+    $invoice->addTotal('Net sum', $user->total_due-$vat_sum);
 
     if ($vat_sum > 0) {
-        $invoice->addTotal('incl. VAT', $vat_sum, false);
+        $invoice->addTotal('+ VAT', $vat_sum, false);
     }
     $invoice->addTotal('Total due', $user->total_due, true);