Add extra fee to Oscommerce

I have a modified Oscommerce site. I need an extra fee to be added. It needs a switch in admin to turn on/off. And it needs to be either a percentage or set amount, chosen by user. This will be set in admin. The name of the fee needs to be able to be modified as well as the fees description. THe customer will get a choice to accept or decline charge, accept is default. THis can be on a new page or on [login to view URL] or checkout_confirmation.php. The fee if a percentage will be a % of the sub total. Fast, efficent providers only.