How to safely add an additional fee for additional services in an online store? [closed]
I have an online car rental store built on Python Flask. My problem is that: When choosing a car, the customer goes to the checkout page, where he can choose additional services, for example: GPS navigator, child seats and other services. The cost of some services depends on the number of rental days, for example, for a Gps navigator, the cost of 1 day is $ 5. The question is, how can I safely get the cost of these additional services from the FrontEnd and add to the cost of the car received from the BackEnd side
Additional services look like this: enter image description here
I am trying to safely connect the cost of renting a car with the cost of additional services, some of which depend on the number of rental days.My task is to implement this system as safely as possible, but I do not know how it can be done