Here's the letter I posted. Feel free to use and modify a copy if you wish to send your own letter.
I have been looking to create websites which I will use to show and share the photographs I take and the artwork I create and sell. If I cannot pay for users to have higher-speed access to download high-quality images, I will quickly lose business.
I have friends who are working to create online games which will be available for a low cost, but with limited and/or unreliable access to their site, these games will never get off the ground. Can you picture a world without Facebook or Google? If net neutrality had not existed when these sites and services were created, it is very likely that they would have been kept out by MySpace and HotBot or AltaVista.
By allowing the backbone providers and ISPs to prioritize traffic, you are allowing them to turn the internet into a series of toll roads. If you do this, do not be surprised if a new, uncontrolled and decentralized system comes in and eliminates this system that allows the government to crack down on large-scale piracy and other commonly agreed objectionable things such as child pornography.
Enforcing some user freedom and network neutrality at this point will prevent the "dark" networks from becoming major considerations in the near future, and will ensure a safer, cleaner experience for all involved.