I didn't find a thread exclusively for NK so pardon me if this has been mentioned.

I designated NK as my airline for the incidental credit for this year. Spirit offered customer the opportunity to purchase bags at a lower price during ticket purchase, so I did. However, the incidental charge would not be picked up by AMEX since it will reflect as a ticket purchase. So just a heads-up. I ended up calling AMEX to have the charge manually credited.