In order to use national, fleet, lease type of accounts or perform third party invoicing you need to link the given customer to the account that you want to invoice.

• Go to POS
• Click on the spy glass right next to customer
• Choose the customer that you want to invoice to third party
• On the "Customer Information" tab you will find the field "Invoice this customer"
• You need to click on the spy glass next to the field "Invoice this customer" and choose third party account
• Click on the green clock
• Click on save