When you preorder a book here on Payhip, you'll pay for it at that time.
Your order will get forwarded to BookFunnel, the service I use to share many files. When release day comes, BookFunnel will send you an email (to the email you used to buy the book) with a link to download it.
They have useful information on how to download it and upload it to whatever reader you use for reading ebooks. (And if you get stuck, they've got people who can help you figure out your specific device.)
If there's any trouble after release date, absolutely drop me a note (celia@celialake.com) and I'm also glad to help figure out how to get you the file.
Once the book has come out, anyone buying it will get a copy of the file from Payhip (download it and send it to your device.)