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: 
Since it does not appear that the site actually supports this post-order modification (which is understandable, due to payment processing), your best bet is to call Amazon's Customer Support - 1-800-201-7575.
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.