+=head2 C<convert_to_delivery_order %params>
+
+Creates a new delivery order with C<$self> as the basis by calling
+L<SL::DB::DeliveryOrder::new_from>. That delivery order is saved, and
+C<$self> is linked to the new invoice via
+L<SL::DB::RecordLink>. C<$self>'s C<delivered> attribute is set to
+C<true>, and C<$self> is saved.
+
+The arguments in C<%params> are passed to
+L<SL::DB::DeliveryOrder::new_from>.
+
+Returns C<undef> on failure. Otherwise the return value depends on the
+context. In list context the new delivery order and a shipto instance
+will be returned. In scalar instance only the delivery order instance
+is returned.
+
+Custom shipto addresses (the ones specific to the sales/purchase
+record and not to the customer/vendor) are only linked from C<shipto>
+to C<delivery_orders>. Meaning C<delivery_orders.shipto_id> will not
+be filled in that case. That's why a separate shipto object is created
+and returned.
+