I think your headed in the right direction with your ideas. Have you thought about large prints, like posters. I just recently added some to my shop. Good profit margin for me, even though shipping is a beast because I would need to make the investment of buying tubes wholesale. Also It might help to name the color of the paper in your descriotions. Some cards look white, but the lighting is yellowish/organgish:
http://www.etsy.com/listing/99318125/vintage-camper-cards-8-white-camperAlso you should consider shipping overseas! It is not that bad. I also ship paper products 8x10" like you and it is very easy. Message me if you want to know more.
Otherwise I would give the final suggestion of using color. I got that advice not to long ago and I'm working on it.
Oh and as for me..
My ideas are to do more prints in different colors
Get a collection of poster size prints going
Think about limited edition Christmas prints