The "infant tax" is BS; but as I understand, it is true that the infant fare should be 10% of the available adult fare at the time of purchase.

Having said that, one time I managed to go AUS-IAH-MIA(CO)-PTY-CLO (CM) with an unticketed (lap) infant. I thought the ticket was issued. Big mess with the "unticketed" infant checking in with CO to go back to AUS (BOG-IAH-AUS). After fighting, including talking to Houston, I was forced to buy the infant ticket on the spot. I thought I was being double-charged; but I was not, my credit card record later revealed. The infant and CC number were in the record ... CO had just neglected to ever collect the fare. (Pity they couldn't explain that clearly at the time, it would have saved a lot of hot air being exchanged.)

Anyway, to the point: I paid a minimal fee for the belatedly bought infant ticket. About $100, including taxes. (Including an abnoxious tax I should never have paid that is only on to tickets bought in Colombia.)

The apparent moral of the story: Infant fares are not set in stone and might be negotiable in circumstances such as yours ... although they did already get it out of you.

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