Don't bother trying to do this over the phone or online as it is more likely to get screwed up than not. The infant ticket is just 10% of the fare, and all the taxes. I live in Guatemala and have a toddler who has flown many times, everytime I did it online, or on the phone the deal got screwed up as they sometimes would want to mail a paper ticket (which ended up in the USA not Guatemala), other times do it on an E-Ticket, but they did not know how to use the eticket in Guatemala and would come down to the gate and I would somehow have to pay a second time and get one of the charges refuned.. Show up a few hours early as it seems they have to reprogram the space shuttle to do the infant tickets, plus look at the binder of adoption papers, pay your 150-200 bucks and be on the way. This way you know you are only paying once, and the price for the infant ticket is always based on 10% of the full fare price, not the discount fare so it does not matter when you buy it.