What is happening is that VAT is being applied to your purchase. We try to apply VAT when someone first loads a book page, and only show the full price including VAT, but very occasionally it is not applied until a customer gets to the shopping cart.
If the change in price is happening, but when you continue to purchase the book you are not seeing sales tax added, please let us know by emailing us at email@example.com.
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Interested in creating your first Leanpub book? Here are some quick tutorials for our most popular writing modes: http://help.leanpub.com/en/articles/3088382-quick-walkthroughs-for-getting-started-on-a-leanpub-book