As Woofgang has indicated, if an item gets lost in transit it's the seller that's responsible for sorting things out, not the buyer. (The law is very clear on that point. If anyone should end up making a loss it must be the seller although, of course, they can then seek compensation from the courier).
Woofgang correctly states that if you've bought through Amazon it's the responsibility of the relevant trader to sort things out but, since Amazon guarantee to fix problems with anything listed on their site anyway, you can still through Amazon customer services to sort things out. (Indeed, I'd suggest that would be the best route).
If you log into your Amazon account and then click on 'Contact Us' you'll eventually reach a way of actually speaking directly to an Amazon employee on the phone. It's a while since I last had to do it but, unless things have changed, you enter your phone number on the site and then choose between being called by Amazon immediately or later. Whenever I've chosen 'immediately' my phone has rung within a minute or so, with a helpful person (in Ireland) on the other end of the line. So you don't even have to pay for the call.
On the few occasions I've had Amazon orders go missing, I've never had a problem with getting the problem quickly resolved through that route.