How to post buy 3 get 1 free When card price varies

[PaperTreasuresShoppe]

Hello fellow card makers! I'm just learning as I go and here's my latest dilema.
My prices vary from 3.50-4.50 so if I post this sale, when setting up I have to put the price for 3 to get 1 free. The only way I'm seeing to do this is to take my average price of $4 and put the price at $12 then in the description say a price adjustment will be made if needed. But I don't know how or if you can make that change after the sale.
Is there a better way to go about this?

[Former_Member]

I've seen buy 3 and put the free (cheapest one) under comments. Maybe a custom listing. I've only seen stuff done with all the same price point.

[PaperTreasuresShoppe]

I've only seen it done with the same price too. I really want to do this, perhaps
I could just put up one that says when you buy 3, convo me with your choice of a free card. That way, I wouldn't have to make price adjustment, which I don't know how to do. Do you see any problem with doing it that way?

[Former_Member]

I would do it that way. convo is good.