P.S. Don't Write Back was the second short story from the book, More Tales to Give You Goosebumps.
David Stevenson is having a great time at Camp Timber Lake Hills. He is keen on his bunkmates but not so keen on their grumpy camp counselor, Sam. Finally, one day David has gotten a letter that Sam mentioned the secretary found at the bottom of a drawer, but his mother has only written to tell him that they will not be visiting him on Visiting Day.
This bothers David greatly, and the following day he gets another letter announcing that after camp comes to an end, he will be sent to live with his Great Uncle John. David is very confused now considering Great Uncle John is 87 years old and lives in a retirement home.
David sneaks to the office to call his mother, but is interrupted by Sam. On his way out, David realizes there is a mistake on the letter addressed to him. It is for David Stevenson from Camp Timber "Lane" Hills instead but David is at Camp Timber "Lake" Hills. Camp Timber Lane Hills is across the lake. David then decides that since the letter just happens to be addressed to a boy with the same name, he will go across the lake to deliver the letters. Unfortunately, he soon learns that the other David is already back from the dead and then has to deal with the other campers, who try to throw him in the fire.
Upon arriving back, David also learns from counselor Sam that the camp across the lake burned down thirty or so years ago. He checks the postmark on the letter to see it is dated to be from 1964. That must mean the other David should've got his letters in 1964 instead of 1995. The letters were 31 years outdated.