I don't advise using a shopping cart in ASP, too many indexing problems and hard to make freindly URLS for your products. Select something more stable and proven like Miva Merchant.
I don't think so there will be problem with ASP shopping cart... My Client is running a nice cart with over more than 500 products and never get a problem in last 2 years... http://www.kapasi.com
I would advise doing it in asp.net because of it's strong update vs. classic asp. Build your Business objects in .NET like Customer, ShoppingCart (List of CartItem), CartItem, Payment, ICoupon (Cart+Concrete Coupon+CouponBase), CouponBase, Concrete coupon classes for diff coupon types that inherit coupon, etc... Then have a Session Manager Object with properties that strongly type these classes. Have your actual master pag, inerheit th SessionManager and have SessionManager inherit the Real MasterPage Class. This way all your objects and sessions are available to you at all times and strongly typed so no need to ever worry That's how I would set it up anyways - and have, and its very successful, heeh
What indexing problems? Never had any problems at all with getting indexing It isnt hard to get SEO friendly urls either, there are plenty of good free url rewriters for .Net if you dont have access to the IIS server to make the changes there