Trade milage points to buy things from the Shop
These Souvenirs are not available for purchase in the Duty-Free shop and must be gotten through other means.
They are unlockable in the Shop.
In order to see the Staff Credits, you must first obtain the Power Star Statue from the Hard course of King of the River. Go to the Duty-free shop and select it on the souvenir stand, say "surprise" into the microphone and the staff credits will begin rolling.