Hi lordman,

This would be a fairly unusual use for MetaMod or Chameleon, but based on the fact that you can use either to run PHP code, then I suppose it's legitimate!

The general approach with either product would be to detect the POST variables, do any processing on them, then perform a redirect.

Again with either you can easily detect whether the user is logged in ($user->id > 0) and perform a redirect to the login page if they are not.

At the end of the day however I would advise you to locate/purchase a proper shopping cart for Joomla that has 2checkout capability. Try VirtueMart or many others that have free versions available. You'll get all the integrations you could possibly need without having to worry about any programming.

Cheers,
Stephen