Adding a payment policy to a specific customer allows you to keep track of any negotiated deals you may have made with the client for future reference.
To do this:
Navigate to the Directory page
Select your desired contact
Select the edit symbol on the upper right hand corner of the pop up menu
Scroll down to Billing & Payment Settings. Here you may customize the client's tax, discount, payment terms, and terms & conditions options.
Press Save when completed and the changes will be in your system for the client's next transaction!