The Voucher

The cashier scanned the items and the woman bagged everything up. She placed the bags in the trolley.
"That comes to £86.52," said the cashier.
"I have a voucher," said the woman.
She rifled through her purse until she found the voucher among a bundle of receipts and pieces of paper.
"What's this?" asked the cashier, eyeing the voucher which was of an unknown kind to her.
"I got it sent to me last week. It was with a whole load of promotional stuff from this store."
The voucher declared itself to be a  'MYSTERY DISCOUNT' coupon.
The cashier shrugged and scanned the barcode.
The till drawer opened and a siren sounded.
A man in a suit appeared.
"Congratulations, you've won a 100% discount and a mystery prize," he said.
"Wow! What's the mystery prize though?" the woman asked.
The man pulled a gun from his inside pocket and shot the cashier in the face.