Originally Posted by
UA vs NW
They will charge you upon check-in.
You might be able to see something like "Due: $19" under your reservation or on the receipt. I had a similar experience in booking an award ticket (MGB-DEN-SFO) using Frontier Miles, after purchase, they did charge the tax immediately but never took those 15000 miles away from my account upon check-in. Upon check-in, you will be asked to complete the purchase (the dues) before proceed to check in.
But they already swiped my card at the desk when I purchased