It seems you booked during The Love Boat Sale.
Here is the wording, this may help
Guests who book during The Love Boat Sale with Princess Plus – a cruise fare that adds highly desired amenities including drinks, Wi-Fi and crew appreciation, or Princess Premier – which includes everything offered in Princess Plus, plus multi-device Wi-Fi, specialty dining, a photo package and Princess Prizes – will receive a $100 shore excursions credit per person.