Hugo: if you follow the link I sent you they will try to show you (on that page) your most recent order. Ignore that and go below to the options.
The first option is Select an issue so you should choose "Other Order Issues"
That evokes a second question which asks
Select issue details at which point you choose "Something else".
At that point the magic screen with three options appears. That shows
Email Phone Chat
Choose Phone. They ask you when you'd like them to 'phone you. I always choose 'Now' because, when you do, the 'phone rings almost instantly and within ten seconds you're dealing with an Amazonian who - depending on the time zone - might be from Australia, Canada, Fiji or Ireland. I've never had less than excellent service from them.
I suspect that what they're doing with the process is forcing people to think about why they want to contact Amazon. It is the main issue with online retailers that if they make the 'contact us' route too accessible people ring them all the time for things which they could easily discover. Not defending them, because I well remember the first experience I had trying to contact them and it was intensely frustrating.