I want to clarify some details on how the Early Access Program (EAP) works because I'm seeing some confusion here in the comments.
EAP is a 7-day period in advance of General Availability (GA) during which domains can be registered immediately (not pre-ordered). EAP is a descending price ("Dutch") auction, meaning that prices start off high and then decrease as the auction goes on. The reason for this is to efficiently allocate domains to those who want them the most, rather than allowing desirable ones to be snatched up by those with the intent of reselling them. It's the concert ticket problem in domain name form.
We are currently in the first day of EAP. Each price tier lasts for the entirety of the day, and then ticks over to the next lower tier at precisely 16:00:00.000 Z. There are price drops over the first four transitions, and then the final three days are all at the lowest price tier. EAP is a one-time acquisition fee; you do not pay that on subsequent renewal. Different registrars choose different amounts for EAP tiers just like they choose different rates for base registration. EAP began today at 2018-05-01 16:00:00 Z and ends precisely when GA begins, which is at 2018-05-08 16:00:00 Z.
The confusion around EAP is stemming from the fact that many registrars are offering preorders for specific EAP price tiers (or GA). So while we're not yet in the lowest price tier, you can put in a preorder for the lowest price tier now, and the registrar will then attempt to register the domain for you within the first moments of that price tier once it begins. Whereas if you buy a domain in the current price tier, the registration goes through immediately and there's no element of chance like there is for preorders. Typically preorders that you don't win are refunded, though some registrars may charge non-refundable application fees.
Separately from all of that, there are also premium domains; those prices are orthogonal to EAP and affect renewal prices as well as initial registrations. EAP fees disappear entirely once we hit GA, but premium fees persist. Again, the intent of all of these is efficient domain allocation. You may be asking "why both?", and the answer is that both pricing mechanisms are good at solving different aspects of the problem of efficient allocation.
For a full list of registrars selling .app, see https://get.app and https://www.registry.google/about/register.html