Adding a cleaning fee or any additional fees to bookings can be setup using the Additional Charges feature. Additional Charges allows you to add required fees that will automatically add to all bookings.
To setup, go to Setup - Products & Reservation Rules - select the product to Edit - Tax/Deposit.
Additional charges are added to each reservation. You can specify up to two additional charges per product. The fees can be a fixed amount or a percentage of the subtotal. In addition, you have the option to add tax to the fee.