I'm using the buy cards mod and am curious to know whether I can make cards only purchaseable and not obtainable through drawing pieces... In a scenario I made gift cards worth 15 gold -- for the concept of it. Would I limit the pieces per turn option to 0 so they couldn't be drawn but only bought through the mod?

In the cards settings, set each card to be 0 pieces minimum per turn and set number of pieces of cards player receive per turn to be 0 as well. Forces to buy cards.