Mystery shop is an in-game event under Luck Royale tab that offers users huge discounts on items. The discount varies and rests assured all users will eventually get a similar offer. The offer varies and is random from user to the user thus a player can get up to 90% while another user can get a lesser discount. The offer is usually valid for only 7 days unless otherwise stated and once it's over, it's over.
If you missed out on the offer or didn't find the offer or item on sale not suited for your preference, there's always another event in the horizon and you'll never know the mystery shop may pop back again in your client.