YSK: You can only select the extra legroom seats if you don't choose the package option, otherwise you'll end up paying again for a seat change
Don't make the same mistake as me. I was booking through the ZIPAIR website and chose the Premium package with seat selection included as that was the only option with both seat selection, carry-on, and checked luggage included. When I went to choose my seat, I saw that the extra legroom seats were crossed out. I assumed this meant it had already been reserved, so I went ahead and chose a regular economy seat. It wasn't until I paid and went to edit my reservation did I realise that those seats actually are available, they just don't show up if you choose the package deal. It then wanted me to pay a second time just to switch to the extra legroom seats...
I ended up just cancelling the whole flight altogether (since I'm flying to/from North America so full refund is given within 24 hours of booking) and rebooking without the package since I didn't actually want any of the inflight meals and amenity kit add-ons anyway. After clicking no package and adding the seat selection as its own add-on, I was able to select the extra legroom seat without paying twice basically. Hope this helps save someone a headache in the future!