It doesn't matter, they won't use the coupon anyway.
My other account is an 11,000 item all card store.
Two times in order to prove points for youtube videos I was making I put the entire store ON SALE buy one get 1 free for weeks at a time. They didn't even have to use a coupon code, just add two items to cart.
Over 90 percent of my orders were still for 1 item, they did not take the free item. In fact I had more people buy multiple items in separate transactions paying full price and paying shipping twice than I did actually take the free item.
A very tiny minority of customers went nuts though and ordered a ton of cards, each one that did it seemed to purposely spend enough money to keep me from shipping them standard envelope.
Do NOT set up a drop down box listing, customers hate them with a passion and they are extremely difficult to manage once created.
You are also listing cards below the "worth your time" level. A 1.99 shipped card nets you less than 50 cents after fees and expenses. Can you both list and ship 30 of those per hour, that is what it takes to even make $15 an hour selling them. By the way, I already know you can't, our fastest guy at the card shop could not reliably even ship 30 per hour (we timed it), much less adding in the need to initially list and file them away.
My card store now has a minimum 2.99 plus shipping price point. I sell half as much as I used to but I net twice as much as I used to with it.
To answer your real question it will charge for the more expensive one if it is a buy 1 get 1 situation.
You can also simply group cards together in lots, sell 4x lightning bolt instead of just 1.
