Payhip doesn't currently have an automated process for pre-orders. They're possible, but require several manual steps.
When I set up the pre-order, I upload a single page file with a reminder that the book isn't out yet, what to do if you see that file after the release date (email me!), and a little other information.
The night before the book releases everywhere else, I upload the final file on Payhip (replacing that temporary file). At that point, anyone who buys the book or returns to the purchase link in their receipt can get the actual book.
I then manually email everyone who's ordered the book with a reminder that the book is out, where to find the download link, and any other relevant information.
Because there are several manual steps, this might take me a little time. My goal is to do it the night before the book releases everywhere, Eastern US time.