It can be tricky to find but it's
here.
Generally,
your account / help will get you there, but the option to contact them doesn't appear until you've exhausted their other options. Eventually, when you choose 'Something else" on the order screen option list a screen appears offering three options. The best one is to choose to get them to 'phone you. It's almost instant and they're really good at what they do.
I've just checked and the way to get to that screen is through the
Your account / Help route, and choose '
Need more help' at the bottom and it produces a screen showing your orders and asking which order it is. It's after that they show you the three options.
If you ordered from a Market Place seller, however, you have to use the messaging option in your account (
Contact market place sellers')