Last time I used Glacier, it was a separate product from S3 and had its own API.
You had to upload pre-prepared "tapes" for backups. You couldn't mutate an existing backup, you had to create a new one. And frequently fetching and/or deleting existing "tapes" (backups) would cost you money (more so than the original cost of the backup).
That meant you couldn't just ZIP it all up, backup the latest version and the delete the previous one to avoid being doubly charged for storage either.
Basically at time of archiving you needed to determine what was already archived and create a new bundle with only what's new, and archive that only. In the same spirit, restore meant piecing together multiple such tapes into a full restore-set.
Absolutely terrible. It was like having traditional backup-software constraints, but none of the software-support.
If Amazon has improved on that now, good for them, but I figured they probably had to if they wanted any users at all.