Hi Kelly, I just launched my store this week, so was looking at this recently. Due to the lower price point of cards I decided to go with standard shipping. It just makes financial sense… for buyer and seller. I am not sure that anyone would want to pay something like 30% extra for a card to go priority.
Having said that, there is a choice to allow customers to select priority for an extra fee - so you could offer this.
Or maybe, if they order 3 or 4 cards you might want to make a good impression and send it Priority anyway (covering the cost yourself).