"It works like this: If I'm looking at Dan Brown's upcoming book 'The Lost Symbol' and want to tweet about it, I take the book's ISBN number (listed on the product page) and put it on the end of amzn.com/. 'The Lost Symbol' ISBN number is 0385504225. So the short URL would be amzn.com/0385504225," according to this TechFlash story. "For other Amazon products that don't have an ISBN, you use the ASIN number (Amazon Standard Identification Number), also on the product page. The tool also works for Amazon Wishlists. You type 'amzn.com/w/' plus the Wishlist ID number, which is usually at the end of a person's Wishlist URL."
With the growth of Twitter and other sites designed for the detail- and context-challenged, Amazon decided that it needed to have its very own URL shortener system: amzn.com, which is more Amazon-friendly than tinyurl.com and shorturl.com.