Consider the following example. A health club normally charges $20 per visit. They run a special of 10 visits for $100. You only go 7 times and don't use the remaining 3 visits. So the health club tells you that you didn't fulfill the contract of 10 visits and you therefore owe them the regular $20/visit rate - a balance due of (7x$20 = $140) -$100 you paid - that is, you owe them an extra $40 because you didn't use the last 3 visits.

How long would they stay in business? This "contract" is no different than an airline contract.