"Dear Parent: I just won the lottery. To show my appreciation to the community, next Monday, i will be giving out a voucher to each child in the school for a fully paid scholarship to the college of your choice. You need do nothing to get this voucher. If you do not want your child to receive this voucher, please sign the bottom of this letter and return it before next Monday."
Any person who has ever dealt with a hundred or more humans would not be surprised to get a few signed letters returned and would be astonished if not one person called for more information.
Some of the signed letters would be from people who thought you had to sign it to get the goodies.