This allows old archives to be deleted, which might help save on storage costs. More copies of more recent versions of the archive are saved.
tarsnap-old-archives --archive-regex='^.*$' --dry-run=true
If all of your archives match a similar pattern, this regex is a good idea. Otherwise you will want to run once for each pattern in your data set.
This will go through your archives and tell you which old ones are likely to be deleted. Note that this will take a long time to run. It's fine.