I purchased a groupon online. Called to make an appointment, the guy said I need to pay another $120 on top of the $136 groupon to get a window tint because blah blah blah my car has nine Windows and I need a tint that's not clear. That's exactly what people paid with their cash price. If you do not want to honor your groupon, don't offer it in the first place.
I do not support dishonest business.