The good news is that there are many other things you can do to support your crazy writer friends, some of them from the comfort of home. I recommend the following five, in no particular order:
1) Write a review on Amazon. It doesn't have to be long, and it doesn't even have to be positive. Just be honest and be yourself. Say what you liked and didn't like. When it comes to online book reviews, silence is more deadly than a bad review. Bonus points if you also post the review on Goodreads.
2) Give a copy to a friend. Movies are advertised on billboards and TV, but people learn about books by word of mouth. If you liked the book, pass it on.
3) Get your book group to read it. Many authors are willing to join the book group discussions and answer questions either live or via Skype or FaceTime.
4) Throw a literary party. Bring together a group of friends and invite the author to read and talk about her book. Your friends will be impressed that you have an honest-to-goodness writer friend, and your author friend will be deeply grateful.
5) Ask your local library to buy the book. Your library's website probably has a form to submit a request. Or just ask your favorite librarian.