Question: How can I remove just one item from my Amazon order? I accidentally ordered 2 copies of a book. I find where I can remove the item, but it completely deletes the item. I just want to remove one of the two. How can I do this?

Answer: 
If the order hasn't been processed yet you can

* remove both books from your order
* order the book again
* merge the two orders into one order.

I've done this on Amazon.de but I guess Amazon.com should be the same.
From the Amazon site

Cancel Items from Your Open Order
Cancel items now. Instructions are below--either print them or use your browser's Back button to get back to this page if you get stuck.

Click the link above and the "Your orders" button. Sign in with your e-mail address and password.
On the next page, find the order you want to change. Next to it, click the "Cancel items" button. If the order doesn't have this button next to it, it's entered the dispatch process and can't be changed--not even by our customer service team.
On the next screen tick the box next to the item you want to cancel, then click the "Cancel Selected Items" button.