The Early Access Period is a descending price ("Dutch") auction. The fee will decrease every day at 16:00:00 Z for the first four days throughout the week-long period.
Ah, looks like the registrars were still coming online when I tried. When I first tried Gandi, it didn't seem to like .app, but now it does and lets me choose "landrush," which I assume is the way to buy a domain during the EAP.
It's only for the current early access period, so if you don't have a named project already I'd suggest just waiting for the period to end then your pricing is steady. We know how to design good auctions but I'm not sure there's any design that'll be kind to purchasers who can't easily evaluate the value of the item to them while still meeting other more basic goals.