I ordered some postcards to promote my Mom’s book. They weren’t that expensive from vistaprint. Actually the cards were free, and I just paid postage. It let me put a photo of the book on the postcard and some text. On the reverse, I put the book’s description.
The plan is to send the postcards out to museums and libraries in Kansas. I know librarians like to have a piece of paper to trigger their book ordering. At least, I always did.