This article explains how to customize the backup package available for your backups:
You are in the Backup > Backup Plan area.
Select the desired package. You can choose either a package with a fixed storage space size or the pay-per-use package.
Please note: If you choose the pay-per-use package, you only pay for the storage space you use. If you choose a package with a fixed storage space size and occupy the entire storage space, you pay $0.12 per month for each additional GB.
Click Customize Backup Package.
The size of the storage space is adjusted.