Join Date: Jan 2003
Location: CTU/TFU(Chengdu, Sichuan, China) - MIG (Mianyang, Sichuan, China) - EWR/JFK/LGA/HPN (Great New York City Metro Area, NY, USA)
Programs: UA 1MM/AS MVP Gold/F9 Gold; Marriott Lifetime Gold; IHG Platinum; Choice Privileges Platinum
Posts: 986
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.