With a little help from CodeIgniter writing such a script is peanuts.
All you need is in EE : entry_id for the product, a price, a title, a little description - all stored in the DB.
- Create a new table that stores session-id, entry_id and quantity.
- Make a script that accepts $POSTS and stores the above values in the DB.
- You can show the user it’s shopping cart by checking on session-id.
- Make him/her register / logon for the checkout.
- Retrieve all lines from the temp table and submit them to a new table together with the user_id to an orders table.
- Add additional taxes and transports depending on their location.
- Send out the emails.
Takes less than a day to code and deploy.
Or I could extensionify/pluginify/modulify my shopping cart ..