使用单选按钮增加和减少值

I am developing an eCommerce system. Let say if user has total of 1200 right. If he chooses fast delivery options 40 should be added to total that means at the end amount should be changed from 1200 to 1240. I have a developed a kind of application for that. I don't know what but the value is not increment. Don't know what but I need a solution, Here is my code:I am developing an eCommerce system. Let say if