Cost-effective long-term archiving?

Nero or CDBurnerXP or whatever. It's just files.
You'll have to figure out a way to split up the 300+GB among the discs... less is more tho.
No need to over complicate it.

They have:

Snapshots: Package many files together and save, download, or receive them via FedEx on a USB hard drive.

Listed as a feature. Source.

I'm pretty sure that that is what you are looking for.

You can download at $.05 a GB or pay $189 for a 3.5TB drive to be mailed to you, unfortunately their refund program doesn't included the B2 plans.

That's by design. You do not give away drives that contained business-data. You usually destroy them.

Yet the business and personal plans will refund you the $189 if you restore and ship the drive back in 30 days

Huh, strange.

Must be a cost thing then. Backblaze seems to work on tight margins. Or again something else.

So could I use one of those to back up a single FreeNAS machine? Because "unlimited" single machine backup for a tiny bit more per month with the refund option would be perfect as long as a FreeNAS box can count as my one machine.

Unfortunately Backblaze personal is Windows and Mac. B2 is the only one that supports Linux out of their products. Rclone is what I use to backup my stuff to B2 and it should work in a jail on freenas