Did he calculate the tax based on the full face value of your purchase, or the discounted amount you paid for it?

FORBES: Social Confusion: How Do Sales Taxes Apply To A Groupon?

If you offer me 100 dollars and one cent a year from now or 100 dollars now, I will almost certainly take the discounted amount, 100 dollars, now.

FORBES: Interest Rates For Dummies: Solving The Interest-Rate Puzzle

"We're paying an enormous amount of money out of our rail tickets for other users to have discounted fares, " says Mr Gummer.

BBC: Ben Gummer MP fights for fair rail fares in the east