Payment Terms can be setup and customized through the + button next to the payment terms field. Clicking the + button will open the Setup & Configure Tool directly to Payment terms to configure one or more terms for use with customers.

The example below illustrates payment terms of payable within 15 days of receiving an Invoice with a discount if the Invoice is paid within 15 days of receipt.

 
When the Setup & Configure Tool is opened click the New Item icon in the lower left screen to open a new set of payment terms.

Fill in the Name, number of days the payment terms are good for, any discount and the number of days the discount is good for and close the setup and configure tool. Once the payment terms have been set to a customer account they will be applied to each Invoice generated.