The post office tidbit will come in handy eventually. Teiai will figure out a way to stop them from making bank books or withdrawing the money and getting caught, but the post office is a different matter.
As a government institution, Teiai cannot lean on it the same way they can for the private sector banks. It's also a monolith that had huge reserves until Junichirou Koizumi liberalised it. For Mario and Chang they probably won't need it since they're not citizens, but Kaiji needs to remember this for later if the other two make their escape and he's cornered again.