Ian you are right!! when i sold Kirby, we used to go door to door offering the chance to win a free voucher for carpet cleaning.
but I posted last week that i was offering money off vouchers in a letter, but when I removed the offer they got more response.
people are suspicious of anything free, look at the tv advert for Holland & barrett they are saying buy anything and get a second item for 1p..... why not say buy 1 get 1 free they must believe charging 1p is more effective