explain one way in which employment laws in the UK might of the profits of Costa shop changes such a Starbucks

An electronics store is running a promotion where for every video game purchased, the customer receives a coupon upon checkout to purchase a second game at a 50% discount. the coupons expire in one year. the store normally recognized a gross profit margin of 40% of the selling price on video games. how would the store account for a purchase using the discount coupon?