This is
"pay-for-A". It's not a seat selection fee as you're still not guaranteed a particular seat. Rather, it's an optional convenience charge to remove the OLCI hassle. Customers are basically purchasing a feature of A-List status ("We'll automatically check you in and assign your boarding position within 36 hours of your flight's departure - that's 12 hours before general boarding positions become available."). I wonder if A-List status itself, or perhaps an EarlyBird subscription, will be available for purchase soon.
I can't say I'm personally thrilled about this, but
WN knows this is something Customers want. In hindsight, I'm surprised it took this long for them to offer this.