You can issue a one-time invoice to a customer who has an existing contract. The invoice will use the payment data of the underlying contract. This means that the customer does not have to enter payment data again or confirm the payment for the one-time invoice and the stored payment method is charged.
To issue a one-time invoice, go to the "Customers" tab and click on the contract ID of the customer.
Go to "Perform Action" in the contract details and then select "Manually Create invoice".
You can now enter all the necessary invoice information in the "Create invoice" menu. If you want to list several invoice items, you can extend the invoice by clicking on "Add Row". Click on "Send" and the invoice will be created.
Afterwards you are forwarded to the "Invoice Details" where you can edit the header / footer, resend the invoice to the customer and download the invoice.