You don't pay for the ticket for infant, just a flat fee since infant doesn't take an extra seat but sits on parent's lap and uses special seat belt clipped into parent's one.
Which can end up being more expensive than regular ticket on rock bottom prices. Every airline has this up to 2 years, some actually charge more so that situation above becomes quite frequent.
One interesting ones that I was curious about ua the “infant fee”. Though I can’t see what exactly that is for.
Is that only if you want to put the infant in the overhead bin?