I am not licensed in your state but I can offer some general advice.
If you sent the check to the regular payment address and not to the corporate address then there is no legal affect to them cashing the check.
Also, you would have had to send the check, with the letter, and perhaps even indicating on the check itself that cashing the check will be considered acceptance of the offer.
You should contact a local attorney to talk about your situation.
Robert KovacsAsk a similar question