How to add a flat amount to an order. Using Drupal commerce module
You should be able to accomplish that using Rules.

Create a product that has the flat amount of 230€ and use the Rules action "Add product to the cart" to add that product to the cart.

To create the rule:


Create a Rule for event "After adding a product to the cart"
Add the "Order contains a particular product" condition to check if the
order already has the flat amount product
Then add the "Add a product to a cart" to add the flat amount product to the order


You could add the same rule for the "After removing a product from the cart" event, so if they remove the flat rate product, it would just re-add it to the cart.