Cheap birthday gifts to give an author:
Write a review on Amazon for her books, if you've read them. It doesn't have to be a great essay, just a few sentences will do. Be honest. Even bad reviews help, because once a book has 20 reviews it will start to be indexed in the site's search engine.
Click the yes button on "was this review helpful?" on another review on Amazon. Your response will help the book get more attention from the site's search engine, and will also encourage the reviewer to keep reviewing books.
Click "like" and "share" on an author's post that her book is featured on a blog or online
article. For example, this one http://thebasementlevelfive.blogspot.com/2013/07/blogger-book-fair-paula-hrbacek.html
Click the tweet, G+ or like buttons on the blog article.
Doing any of these things is better than just saying "happy birthday". It's actually making the birthday happy.