Calculation Expression

I have looked but have not found this question and I am quite stumped. 

I am creating a form that provides a summary of our services and costs. If a client requests that our documents be sent via overnight delivery, we add an $8.00 handling fee. On our form, we show the cost of overnight shipping. Next to that entry, the form says "+ $8.00" The box next to that is what I am concerned with. I need an expression to make it show $0 when there is no request to send docs via overnight delivery, but when overnight delivery is requested, for it to show the sum of the cost of overnight delivery, plus 8 bucks. HELP!