Where to write a formal complaint?

I would like to write a complaint for the following reason:

I brought my daughter's Macbook to repair (broken screen) in Apple Les 4 Temps in la Défense, Paris. I accepted the quote of 479 euros (It was just an estimation of the repair cost). When I recovered my repaired Macbook today, the assistant at Genius Bar told me that the repair was in fact free, covered by Apple, very nice surprise. I signed the receipt (Confirmation de service au Genius Bar) showing 0 euros due (I have got this paper on my mail). I was about to leave when he said he will double check something. Coming back with the manager, I was told there was an internal error and that I had to pay the 479 euros. I tried to argue that they could not change their mind like this and that they should have made a commercial gesture, it was their mistake. But I had to pay to get my computer. Not so nice.

The computer was just over 2 years old, so not covered by warranty.

To be noted that there was no impact on the screen, some vertical lines appear on the left of the screen but my daughter continued to work with these lines for a while. Until Christmas, a small pression on the screen (on those lines) created some horizontal lines and the MacBook was no longer usable. I think these screens are quite fragile (I know there is a class action on this subject in the US). And I think the cost of repair is too high (nearly half of the MacBook price).

I still love Apple: we have 3 MacBook in the family, a iMac and owned I don't know how many iPhones :-)