Ouch.
I had a wonderful TIF (which I paid for) and immediately signed up for lessons.
Time passes and third attempt made to schedule said TIF. Response from school, "uh sorry, that certificate has now expired, however we're happy to take more of your money and try again"
If a company has an expiry date policy, it should be clearly marked on the voucher and explained to the customer. Surely the average school isn't selling five hundred of these a day?
There should be enough time to explain the house policy. Legalities aside, that's just good customer service.
I was involved with a non aviation business that regularly offered discount and gift vouchers; the terms were clearly printed on the voucher and our staff made every customer aware of them.
We never had any complaints.