Stuart Stone
Pick your favourite strategy Gail:
Isn't it an offer to contract?
A BIG 'No contract, opened in error, return to sender' written diagonally across the envelope in RED.
Conditional Acceptance, with your terms (eg: $50.00 for every letter opened, $100.00 for each response, $150.00 for any cheques written & $400.00 per hour or part thereof...if I receive any further correspondence, then it is deemed tacit agreement between the parties that they accept the terms)
Notice of Mistake...the slam dunk
It probably all depends if you want to just make it go away, or for shits & giggles see what response a low key approach gets, then up the ante until you reach your outcome.