So all you want to do is backup the important invoices?
1 GB of space would be more then enough for you, an invoice should be a few kilobytes.
Some backup programs will zip it up in their own formats and save it onto the device you specified, that could be a USB stick, portable Hard Disk or DVD Disk (make sure too but in a blank before the backup starts).
It shouldn't take too long to do as well I would say about 10 minutes at most.
With data that small you could get away with full backups.