Credits are how claiming works. List a bundle and you earn credits, then use those credits to claim a bundle from another family. You can also buy credits if you'd rather not wait to earn them. They're not money and they don't get spent like cash, they're just how the swap moves.
Credits don't expire as long as your account is active. List now, claim whenever the right bundle shows up. No pressure, no use-it-or-lose-it.