asp.net shopping cart needs fix

We have a web site which is built in asp.net vb & c# and with the use of dreamweavers internal asp.net functionalities. The db is MS SQL 2005. This web site contains a shopping cart. Our bank wanted to test our cart system and they faced an error which we were not able to reproduce. We want you to investigate the reason and reproduce it. Explain us the error and the reasons. After showing us the reason, you can start fixing it, test it and deliver it. You have to setup your own testing enviornment. Which is basicly an asp.net development server and ms sql 2005 server. Please do not bid without downloading and reviewing the site files. You can find them attached.