I do backup once a month to a ‘My Passport’ drive but daily seems too much hassle.
I hear ya, that was my situation too, for years. I realized I wouldn't be happy losing a month of work, so I'm upgrading my backup situation.
Backing up daily is pretty easy once you're set up for it. Just leave your backup drive connected to your machine, and use an app like SuperDuper.
SuperDuper creates a fully bootable copy of your entire setup on a 2nd drive. If anything explodes on your main drive, the 2nd drive is right there ready to go. All that's needed is to reboot from the 2nd drive.
On my machine (ten year old iMac) it takes about an hour to backup around 500GB over Firewire (old connection method). It takes just seconds to launch SuperDuper and start the backup.
You can set SuperDuper to erase the 2nd drive and do a full backup from scratch (takes about 4 hours here) or do a Smart Update which will only transfer the files that need to be moved to make an exact duplicate of your main drive (takes about 1 hour here).
Yes, there are good reasons to update OSX, and good reasons not to. To me, just one somewhat controversial opinion, new and free isn't a good enough reason to update OSX. A good reason might be if you can't run some software that you want to use on your current system.
Opinions will vary on this, so don't take anything I'm saying as the final word. I'm definitely not a backup expert, or a Mac expert. See Cory for that.
I'm not doing a good job of answering your actual question. Do you have the eight years of invoices on your backup drive?
I'm sorry, I have no knowledge of Paperless Invoice software. Is there a forum for that software, or support?