There isn't a customer extranet, as such, right now.
What there is is the capability for a customer to identify themselves at the start of the booking process - to save them from retyping all their details in (if they have previously booked, and they know their login details). Also then TourCMS will ensure the new booking is attached to the existing customer record (rather than creating a new record) - just like it does within the staff facing booking process.
We have a plan to introduce more of a customer extranet functionality - in 10+ languages (!). First step will be for the customer to be able to login and answer questions (e.g. supply additional details). Secondly they will be able to (with a payment gateway) make a balance payment (and see outstanding balance). Thirdly, perhaps download predeparture information (e.g. RTF style documents)