diff --git a/ordernumbers_woocommerce.php b/ordernumbers_woocommerce.php
index 82a2ff4f2ad85bf7a75afb8bc55588ddeea4dcf2..df655f9356a65008e6a3ce40fa70ce2e2edcceb8 100644
--- a/ordernumbers_woocommerce.php
+++ b/ordernumbers_woocommerce.php
@@ -65,6 +65,9 @@ class OpenToolsOrdernumbers extends OpenToolsOrdernumbersBasic {
 		// Custom table widget for custom variable definitions: Hooks for creating and storing values
 		add_action( 'woocommerce_admin_field_ordernumber_variables',    array( $this, 'admin_field_variables' ) );
 		add_action( 'pre_update_option_ordernumber_variables',          array( $this, 'update_option_variables'));
+		
+		// SUPPORT FOR BUILT-IN PLUGINS AND PAYMENT METHODS:
+		$this->paypal_invoicenumber_init();
 
 		// THIRD-PARTY PLUGIN SUPPORT
 		// Install hooks for third-party plugin support:
@@ -330,6 +333,18 @@ class OpenToolsOrdernumbers extends OpenToolsOrdernumbersBasic {
 		$reps = apply_filters( 'opentools_ordernumber_replacements', $reps, $details, $nrtype);
 	}
 
+	
+// PAYPAL SUPPORT
+	protected function paypal_invoicenumber_init() {
+		add_filter ('woocommerce_paypal_args', array( &$this, 'paypal_arguments'), 10, 2/*<= Also get the order object! */);
+	}
+	
+	public function paypal_arguments($vals, $order) {
+		if ($this->invoicenumbers_activated()) {
+			$vals['invoice'] = $this->get_or_create_number($order->order_id, $order, 'invoice');
+		}
+		return $vals;
+	}
 
 // THIRD-PARTY PLUGIN SUPPORT