How to use a coupon ?

[i recup the ball]

I currently own a -90% portal 2 coupon but still don't get how to use it.
Either in my inventory or during the whole buying steps there is absolutely no button to use the coupon.
I checked the FAQ which says the coupon will be used automatically but until the final payment it always show me the 19.99€ price and it never mention anything about coupons.
As I know the no refund steam policy, i'd prefer to be sure that my coupon will be used.

How am I supposed to do this ?

[LoTekRabbit]

When you add your game to cart and go to check out.

The window that says 'purchase for myself' and 'purchase as gift.' Look at where the game is. Right underneath it says something like "add/remove coupon."

[Lab-Rat]

ok ok GO TO THE SHOP, SEE THE GAME, PUT IN THE CART AND, THERE IS A OPCION THAT SAY: YOU HAVE A COUPON FOR THIS GAME, DO YOU WANT TO USED ?

[i recup the ball]

Here are some pics that show there is absolutely no use coupon button or mentionned coupon during checkout.

http://jepix.fr/images/coupon1j.jpg
http://jepix.fr/images/coupon2j.jpg
http://jepix.fr/images/coupon3j.jpg
http://jepix.fr/images/coupon4j.jpg

[Kiya]

Steam -> Exit at the top left of the client, restart Steam, then retry.

Depending on when you earned the coupon, it could just be that it is taking the Steam servers a while to update with new information.

[Marble]

This is what it should look like:

http://i.imgur.com/P9VfRxd.png

[i recup the ball]

Still doesn't work.
I'll wait a bit to see if the issue will get resolved in the next following days.