Amazon links can be very long indeed. You want to be able to turn this:
How to use amzn.to? Amazon has signed up with bit.ly and reserved the amzn.to domain, so that if you use the bit.ly service, your shortened link will reference amzn.to, which redirects to bit.ly, is translated back to the long url form, and then redirects to Amazon.com.
Disclosure: Goodies for Geeks is an Amazon affiliate site.