How can I prevent users from overriding the total cost in a shopping cart, when submitted as a hidden input field?

Question

I'm having serious problems with accepting payments.

I'm passing the total amount in a hidden field

<input type="hidden"
  name="checkout-flow-support.merchant-checkout-flow-support.shipping-methods.flat-rate-shipping-1.price"
  value="129.00"/>

Some of the users changed this value to 2 using firebug and submitted the form. Instead of getting $129, we only received $2.

I have no idea how to proceed this anyone help me quick .