If your book is coming out immediately in paperback, sure, you can make bookmarks. If it's not, though, bookmarks don't make much sense. As for postcards, I've done them. Do I think they work? It depends on what you're going go to do with them. If your local bookstore or library will keep a stack of them to hand out to readers, or if you're going to a convention, they might work. But I will warn you- paper swag is often thrown away.
Vistaprint is where I get my postcards, and I've heard you can do bookmarks through them by putting the images side by side on a postcard and cutting them yourself. Whether or not you want to do that is up to you. I believe GotPrint does bookmarks, too, and has reasonable prices.
Does that answer your question?