Total Sum Of Retained Backup
Using this feature will ignore hard-links when calculating backup snaps size. Each snap will be calculated as a unique backup. Given that you are using incremental backups - An account with 1GB, with 10 snaps will probably consume 13 GB of disk space inside your backup destination (10GB Source + 30% incremental changes). Using "Total sum" will count 10 snaps as 100GB (10GB per snap X 10 SNAPS).
Average backup size:
Choose this option to charge your clients for the average received disk amount (Summaries all backups divided by the backups number). For example, An account with 10GB & 10 SNAPS will be calculated as 10GB (10GB X 10 SNAPS / 100).