Wallet holsters are illegal under Federal law; a gun in a wallet holster is classified as an NFA firearm, and must be registered with the Feds just like a full-automatic weapon or a suppressor.
Here is the word, straight from the United States Bureau of Alcohol, Tobacco and Firearms: you'll see the passage on page five and continuing on page six.
Sorry about that. I don't want my gun wrapped in anything when I fire it, anyway, myself. I want it firmly in my hand.