[vasilis]

• As I described in my previous post,
  

  This additional charge is programmatically calculated for each product in the order, depending on the shipping address and the price of the product

  Not all products require this additional charge, this is defined programmatically, which ones.
  So, the hidden product solution would require, just before adding the hidden product to cart, changing programmatically it's price, to be the same as the sum of the additional charge per product in the order, is this what you mean?

• On the other hand, dealing with the order functions is a more 'clean' solution as long as I make sure that the additional charge amount is passed through to the cart total amount and stored in the order record. Also, I need to display this additional charge in the screen invoice and email notifications as a separate charge, below the tax amount.
  
  By the way, would you happen to have any comments on my previous post, regarding the order functions and variables that I refer to?