Snapshot rotation with Amazon S3


Backup to amazon S3 bucket works well (thanks !)
I have a question about backups rotation.
In the snapshot config, I set "Maximum number of local archives" = 3
So I can see a maximum of 3 archive files from the WordPress backend.
But in my Amazon S3 bucket, all archives are kept.
Do you have an advise to force backup rotation here too ? (because costs will apply if all my backups are still conserved !)