On Linux, there are two ways to set up a printer in PostalPoint. PostalPoint can write directly to the label printer's USB connection. If the same printer is also set up as a system printer in Linux, printing anything from another program makes the Linux print system (CUPS) take over the device, and PostalPoint loses access until the printer is unplugged or the computer is restarted.
The fix is to have PostalPoint send its print jobs through CUPS as well. In Settings → Printers, choose the printer type that ends in "via OS print queue" and select the matching system printer. Save and print a test label.
Note that the Linux/CUPS printer driver doesn't need to be correct; PostalPoint instructs CUPS to bypass the system driver and relay the label data as-is from PostalPoint's built-in drivers.