Even when you accept a bill payment outside of the Zen Planner online payment processing system (cash, paper check, Paypal, etc.) you can still track it for revenue reporting purposes by processing it as an offline payment in your database.
Navigate to the member's profile that you need to record the offline payment for, and in the submenu, click on the Financial link. From there you can click on the Make a Payment button in the upper left hand corner.
Select the bill(s) that you need to pay, then make sure that you have selected Offline Payment in the left hand menu.
Click on the Make a Payment button to process the offline payment. When you go to Financial> View All Payments in the member's profile, you will see the offline payment listed with all the other payments.