From experience in running various marketing campaigns, the magic 'number' is 1 in 10. Charging $X with the option of paying $1.1X (or just over, even) for a 0.

From experience in running various marketing campaigns, the magic 'number' is 1 in 10. Charging $X with the option of paying $1.1X (or just over, even) for a 0.1 probability of getting the money straight back will get the most people buying into the scheme. That'll be around 1 in 10 people too. Force the extra payment and around half of them will walk away there and then(1). Bump the price up to $1.1X without mentioning it, only that there's a 1 in 10 chance of getting it free and you'll see the overall take-up go up anywhere from 25% to 100%. Long story short: Make your new price $1.1X without fanfare, offer a 1 in 10 chance of getting it for free and you all win. They get their chance at a freebie at a cost they don't notice, you get your promotion vibe bumping up sales. (1)I know that that's obvious, and it was dumb to run it - but we ran it anyway for the sake of completeness.