How do I create or edit the message customer gets upon completing payments?

[Racerdad36]

I want to provide a message to customers' confirmation of payment.  PayPal instructions not evident on how to do this.

[Snow-Cat]

One option is use PayPal's IPN or Instant Payment Notification feature - see this info for an explanation.  

Another option is the use of Auto Return - you can learn more here.

And there are 3rd party services that have the features and/or functions you have in mind.  For example check out this one.