As has been discussed ad nauseum in the past in the Avios thread: it can be done over the phone or at the airport. Taxes should be around $17. However agents over the phone have difficulty doing it, will tell you to do it at the airport. Agents at the airport also have difficulty doing it, so leave yourself an extra 1/2-1 hour. May be worth it just to pay the 900 Avios and not have to worry.