This is an incredible deal!
Note though that it is for their new "economy light" fare which includes ONLY a carry on with no option to buy up later (AFAICT). A better option for most would probably be to add the "bundle" up front, which adds a checked bag up to 50lbs and personal item up 22lbs. The total price for that is $299, still an incredible deal.
It may potentially be bookable with BP as well via JetBlue.