title: How to keep PayPal variables secure
body: <p>I'm trying to link my 3rd party shopping cart to PayPal, and in the process I'm finding that my variables are extremely exposed, such that, if someone uses, say, Firebug, to manipulate the values of my variables, they can change the cost of the items in the transaction.</p>
  
  <p>I'm very new to online carts and shopping, so my question is how do I keep this layer of exposure away from users and secure for the website?</p>