Nevermind, I messed with their demo for five seconds and it sucks already. Tried to give you a quick answer and I struck out 😉
Basically, you just need a secure site, get familiar with using sessions (to maintain state for the cart items), or use your database to store the cart items (allthough this can be very taxing on your system). Since php will append the session id if it cannot write a cookie automatically (have to turn on session.use_trans_sid . Only use sessions in this way for storing shopping cart information as it can be a security issue), sessions are ideal in PHP for shopping cart info. You can also store arrays as session variables, which I think is pretty convenient for carts.
There's another quick answer for ya!