It should be 10% but it could be either that’s the cheshbon of what they actually price as the cost of the airfare as opposed to all the other charges that are included in the ticket or it could be a mistake in which case you got lucky, or as happened to me once, they only charged you taxes but not airfare and this your ticket isn’t paid for yet and they won’t let you fly. So you should check online and make sure that your baby is fully ticketed.