You only have to pay for Economy Comfort (front of cabin), exit row, and "seat in a row of 2" seats.

It may be that all the regular (free) window seats are already selected...seat selection can be done at any time after making your booking...in which case, you will have to pay for any of the remaining window seats in the paid zones. But carefully check the entire cabin before deciding there are no free window seats left. You must scroll through the cabin to view the entire economy cabin.

You are not charged to select a seat in advance - unless you choose to buy one of the Economy Comfort/Exit Row seats. Your fellow passengers have been selecting seats on this flight for free for the last year or so.