The "purchase on account" option is handled by a company called PowerPay, which is an independent payment service provider and not Lockenkopf. The invoice will be sent to you by email on the day of dispatch.
Please transfer only with this invoice to PowerPay and not to Lockenkopf.
For outstanding invoices, PowerPay automatically sends an invoice by post after 30 days, but this is associated with an additional cost of CHF 1.50.