Setting up a fee item and selecting the option of Voluntary gives you the ability to prevent that amount from showing on a member’s outstanding balance due.
Future statements or account balances will exclude this amount unless specifically included at the time statements are created or balances are viewed.
If you are on an accrual basis and need to recognize voluntary fees only when they are paid (i.e. prevent them from reporting as an outstanding Accounts Receivable balance in your accounting software) you can contact support to have this feature enabled.
NOTE: Voluntary Fees that are assigned to a member on the Members Account tab will appear on invoices as part of the balance due. Invoice templates can be created that would omit any voluntary fees. To do this go to Setup->Billing->Template Manager, and once in there select Invoice as the template type. As you are building the template there are data fields that can be selected that will omit any voluntary fees from being included.