Generally speaking, you should never have to pay for a book endorsement. Also, if you ask for a book blurb and an author gives you one, be sure to use it. Check with your publisher in advance to confirm that the publisher is willing to put a quote by your potential endorser on the book or website. The worst thing you can do after you get an endorsement is not use it. When the time comes, be sure to return the favor by promoting the contributor on your own social media pages. You might even want to send a thank-you gift. And remember, someday in the future, a new writer may ask you for a book blurb. Pay the favor forward and graciously agree to write a book blurb.