Your first option is to keep the money and send a note thanking them for the generous check.  You can state how surprised you were when you received the money.  Remember, the check may have been their way of saying thank you and acknowledging that your hard work and kindness was much appreciated.  You can write something to the effect that the next time they need your product/service, it is on you!

Your second option is to return the check along with a thank you note.  You can state that although you were touched by their overwhelming generosity, you just couldn't possibly accept money from them.  Let them know that it was a gift and you were delighted to be able to share it with them.

Lastly, you can donate the money to charity and let the person know this in the form of a thank you note.  Make sure that the charity you choose is something they would deem worthwhile.  Explain to them in your note that you didn't expect payment and you felt that donating the money would be a great way to say thank you for their generosity.

More Quick Tips:
Business Telephone Calls	Proper Introductions
Email Etiquette	Business Meetings in the Home
Are You Busy?	Life In Your Home Office
Unexpected Payments: To Accept or Not to Accept	When Your Healthy Home/Work Life Balance Is In Jeopardy
Dealing with Difficult People	Equal Share
Returning Phone Calls	Handling  Little Awkward Situations
Call Waiting Etiquette	Should I Take The Blame?
Networking Dilemma	It’s Not Your Job Title That Is Important….