To add a payment term in a quote:
In the left Navigational Column, click the Sales tab and select Quotes.
Click the New Quote button.
Select the template from the list on which the new quote will be made.
Note: You can create your own template for any quote.
For more information on how to create a new quote template, refer Creating Quote Templates
Under the General section, select the payment term from the list in the Payment Term drop-down box.
Click the Save button.
The payment term is set in the Quote.