ASP.NET has a HiddenField control that does exactly what it's name suggests, so I'm not sure why it would be an issue.
It really sounds like there is something else going on here though. How can you have a per customer shopping cart but not be able to handle some additional bit of information per customer? Even if there is some technical reason why they can't use a HiddenField control in the application framework, it should be possible to add the HTML hidden field to the text of the HTML directly. It would be ugly, but it would work.
As for being much harder, there is really not such thing as a trivial ASP.NET application. There is a lot of setup and tear down in even the simplest of applications, and most .NET frameworks make it worse.
Jay