Protection of the site from the substitution of input fields. How to implement?

Question

Ruby on Rails

Good day. There are input fields on the site that are checked only by means of JS and then cleared by filters in the PHP script, but almost everyone knows that a POST request can be sent not only by clicking on the submit button :) The most important point for me is the substitution of price and quantity items. That is, an attacker, or a freeloader, can send 1000 items to the script at the price of 1 kopeck and the script eats this business without blinking an eye, redirecting it to the payment page. How to be? Is there an easy implementation?

Answer 1

Rule of thumb: don't trust user data. On the server, get the price and other necessary information by the product ID.