This is a good example of thank you letter replying to someone who send you an appreciation letter. You have to modify the letter according to your purpose.
Dear Mr. Alex,
I would like to mention my heartfelt thank you for your kind support and the appreciation letter you sent for our "Save Environment Campaign". We are very much pleased and encouraged by your appreciation letter.
We are working for last four years to make people aware about environment protection. We also encourage people to use non-pollutant products or products that do not damage natural resources. Our " Environment Safety Campaign" has been effectively running throughout the state, and we are planning to spread this program across national level.
To serve this purpose, we just want people with a wish to work for nature's preservation and protection. People who are voluntarily ready to participate in our programs and people who can contribute their moral support to our fight will be definitely inspiring for all of us.
We are happy to invite you for our next campaign at Town Hall on [date] when all other leading organizations and supporters will gather to share their ideas about environmental problems and their solutions.
Your presence will be an honor for our program, and it will give a message from both government and environmentalists. Thank you once again to appreciate our campaign. We look forward hearing from you government's participation in this program.