I'd probably add a column called XLPrice to the cart, then update the price calculation to:
TotalPrice = (([Size]=="2XL")?[XLPrice]:[Price]) * [Quantity]
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.