Ronnie Hyslop

Hello, I have built my first e-commerce site from the knowledge gained from this book. This book has played a monumental role in the success of the site. The client is now wanting to allow the use of Promo Codes, on a per product/per cart/per categoery basis etc. I cant get my head round the problem. Some pointers would really help me as I cant think how to add the promo code to the cart and update the price of added products after the promo code has been added etc. The Promo Codes will exist in a Vouchers table: Please Help Ronnie