Ultimately you have to give the collection agency a motive to do something for you. They usually won't do anything unless they're really motivated.
Sometimes contacting their regulatory body... IE consumer protection will be the stick that you need to get your letter.
It always puzzles me that a collection agency won't give you a receipt for a payment.
Collection Agencies are required to keep strict accounting of what they collect. So they must have record of your payment and should be able to give that to you upon request.