Versendet die Rechnungen an den Kunden, wenn die erste der Lieferungen, worauf sich dieselbe Rechnung bezieht, erfolgt.

shipment_success.rb

# Shipment.rb
InvoiceMailer.sent_invoice(self.order_cart.invoices.last).deliver if self.order_cart.shipments.one?{|s| s.state == 'shipped'}
InvoiceMailer.sent_invoice(self.order_cart.invoices.last).deliver if self.order_cart.order_items.one?{|b| b.state == 'done'}