Originally Posted by
ordsky
if it was an international leg you do get an infant boarding pass that has NS printed in seat column thus making sure the tax has been paid in addition to the wording plus infant on your boarding pass
Data point - I can confirm as my son and daughter in-law have to do this with their 1 year old daughter when traveling US --> Brazil. Even though she sits in lap, there is a 10% collection. Interestingly, I got them upgraded to J on JFK-GRU and the 10% was based on J fare.