A variation of this hack which is used by my local car wash: Buy a booklet of 10 pre-paid vouchers for the car wash and get a free car wash ... right now.
According to the owners, moving the 'free' car wash to be immediately available rather than at the end of the voucher book say a significant increase in people who bought them - even though ultimately it made no difference financially to the customers.
It's exactly the same thing as selling someone 11 pre-paid car washes. But by making the 11th car wash "free" and available immediately people perceive that as a bigger boon than a 9% discount.
According to the owners, moving the 'free' car wash to be immediately available rather than at the end of the voucher book say a significant increase in people who bought them - even though ultimately it made no difference financially to the customers.