Originally Posted by
MSPeconomist
Every lap infant needs a ticket for international (Canada etc. might be an exception) travel, but the lap infant ticket costs 10% of the fare plus 100% of the fees. If you don't buy it at the same time as you purchase your ticket(s), the price could go up.
That makes sense, so basically they are just distinguishing between a ticket (required internationally, albeit at 10% of full fare for a lap infant) and a seat (which would cost full fare but not required below 2 years). I guess I just always assumed even a free lap infant flying domestically would still be "ticketed" for $0. Still doesn't help that the website seems unable to accommodate that mechanic.