$9 sale tickets and most other promos like $35 off for 35 hours are only valid online. So you have no option to avoid the fee.

The concept of LCC fees should be to encourage people to use the cheapest way possible, which is booking online. this will try some pax to book tickets at the airport.

I would rather NK impose an airport check in fee. This is something that anyone who knows the rules can avoid. Yet it collects revenue for the airline from those who don't which helps to keep ticket prices low.