PostalPoint can share one database across every computer in your store, so all registers see the same local data (mailboxes, merchandise, transactions, and so on). There's no per-computer charge.
Pick one computer to be the host; it keeps the default Internal database. On each additional computer, go to Settings → Databases, set the database type to "PostalPoint Host PC", and follow the Local Network Connection instructions shown there to connect it to the host. If you have problems connecting the computers, check for a firewall and configure it to allow PostalPoint and/or allow inbound connections on TCP port 7678.
The host computer needs to be on whenever the other registers are in use. Customer accounts are also synced through PostalPoint's cloud service, so they're available across computers either way.