Calculation in code behind

Question

Chauffeur service cost $30. It is an option to user whether they want the service or not. When they already select car, and day of rental, I store it in session.

Answer 1

When you provide to select the service in a dropdown, why not store that status in session as well ? Like

bool chauffeurService= YourServiceDropDown.SelectedValue=="1" ? true :false;
Session["chauffeurStatus"]=chauffeurService;

Then inside your form where to process values

// Check if Chauffer service status is 1 add 30 dollars else don't add anything