Dynamic Shopping Cart - Best way?

I am currently working on a site that has a dynamic shopping cart. There is only one product being sold, however there are several variables (size, color, style, design, etc.).

The desired result is to have the user go through steps to get to their final product. For example, you are presented with the color options. After you click the desired color, you are then presented with the next set of options. Once you complete the steps, you are then taken to the shopping cart with the product and you can order via PayPal and/or Google Checkout.

I am curious to see what people recommend as the best way to have the selected variables carry over to the next step, and then the final product.

Thank you for reading

Definitely use session variables.

As of now, I am planning on using PHP Sessions to store the data. Each set of options will reside in an HTML form, the user selects, and the data is stored in the session.

I will then have a PayPal button that calls the information.

This is my game plan, but I am wondering how to go about structuring it. How should I structure the user input forms? Also, if anyone has experience with the PayPal buttons, how do I call the data into the correct field? There will be several options and I am slightly confused on which PayPal button to use.

I realize my questions may be slightly vague but any ideas, suggestions, or references are appreciated, as is your time.