I had someone set up an amazon.mx account using my credentials somehow.... they ordered a bunch of online books, include a $1200 textbook. I have no idea how they were able to do it.

I was able to get them to reverse all the charges pretty quickly, but I still get messages from amazon mexico saying i should buy stuff, and for a while amazon.com would ask me if i wanted to go to amazon mexico when i logged in.

I have only been to Mexico a few times in my life, and my spanish is really bad. I have no idea how it all happened, since they didn't seem to be able to (or didn't) change any settings that would lock me out of my account.

As a sort of counterpoint, I really wish that Amazon UK was integrated with the Amazon US customer database. I'd like to be able to log into Amazon UK with my normal credentials and order something from there, with the option of international shipping - because occasionally I want to order a product that's not available over here in any way.

It works this way for me. My credentials work for both .uk and .com although I only have Prime on .com. My order history is unique to the country site, but address book is the same for both.

Are you sure you didn't create accounts on both with the same credentials? That's what I did.