im not sure how to word it, but i've had to do it several times,
it's not "checking" a bag, but on several occasions they've asked people to give their bags to them at the gate, they put a pink tag on it and when you get off the plane they immediately give it back to you, they often do this when there is limited overhead space.
Anyway: If you bring transpack as carry-on, you will not have to pay extra.