I was looking at a book on the uk Amazon site. It had a preview you could click and read. I usually use amazon.com (USA site). But when I went there that preview wasn't available. So I just went back to amazon.co.uk, clicked the read preview button, and of course because I wasn't logged in it asked for my login details. I login, and then receive this message: "Thank you for transferring your device to Amazon.com.au"!?
I want to keep using amazon.com (USA site), not .au - what have I effectively done? And how do I change it back to the USA?
You will need to contact them if you notice any problems, but you should be able to log in and change it back to where you want to. I did the same as you ages ago, and then lost my access to my Family Library, which they don't offer on the AU store. So I had to contact them and get them to reset everything and then I could fix it all up again.
If you can go into Manage your Content and Devices, and go to Settings, I believe you can change back to the US store. Under Country it should say Australia, but give you an option to see what other Amazon marketplaces you can use. From here you should be able to switch back.