Once listed live, nothing can be changed on the add.
If you end it to relist with the new price , you'll still be charged the original £19.99 ad listing fee, plus another £19.99 for relisting the vehicle.
As advised --- let the listing run to 30 day completion; just continue to Negotiate an acceptable selling price and only remove the Ad. when you have a definite buyer with Cash in hand or a Bank Transfer is safely deposited/confirmed before handing over Vehicle keys/documents on buyer collection. No further FVF selling fee is charged.