There is a tt_products 2.6.3 already under development and available to the sponsor who have made an order with payment under http://ttproducts.de .
On TYPO3 TER you can download tt_products 2.6.2 . You must click the checkbox in the EM (Extension Manager) to allow all extensions without securty check. Then you must update the list of extension in the EM.
The tt_products versions until 2.7.0 do not have the new feature to store the orders as hidden on the payment page. This will be introduced with tt_products 2.7.1 .