I need to find companies that will print and mail letters for me. Each
letter is different in its content and sent to different addresses. I
need to be able to send the letters in any quantity at any time and be
able to submit these orders to the mailing service electronically over
the Internet. The best answer will list the top 3-5 companies
providing this service.

I am not looking for a direct marketing mailing service that only
provides bulk mailing of identical mailers. Each piece of mail I will
be sending has unique content and this type of service will not work
for me. At any one time, I may send in an order for just 1 or even
500+ individual letters.

You might find this previous question helpful:

http://answers.google.com/answers/threadview?id=446892

I hope this helps.

Have a good day.