If you purchased your ticket by telephone or online, the original receipt would have been mailed to the address you provided.
If you purchased the ticket at a Delta counter in person, the original receipt would have been provided to you at that time.
If you purchased the ticket through a travel agent, the TA would issue you any relevant receipts.
You are essentially asking Delta to give you a DUPLICATE receipt of what they have already provided you. I don't see anything wrong with them charging a fee to cover their administrative costs.