What is the ruling on taking payment for delivering money to another person while misleading them into believing that someone other than myself will deliver it?

You must repent to Allah for what you have done, for cheating, cunning, and deception lead to Hellfire. The fee you took is unlawful for you, as you were merely an agent to deliver the amount, and an agent is not entitled to take anything from the principal except with his knowledge. You must return the fee to its owner by any means, and you are not required to inform him of what you did, but you would be absolved if you informed him and he forgave you the amount.