Can u buy iCloud storage as a one time purchase?

No, the charges are billed monthly: iCloud storage plans and pricing - Apple Support

There is no cloud based computing service that sells permanent storage. All cloud services and storage (Apple’s iCloud, MS’s Azure, Amazon Web Services, Dropbox, Box.net, MS Onedrive - every cloud based community service and storage) are leased or rented with some recurring payment schedule.

The basic reason is storage is not a firm fixed cost commodity. It continually costs (hardware maintenance and repairs, electricity, cooling, etc) the company providing the cloud storage to continue to provide it, so they need revenue to keep providing it to you. And cloud based infrastructure is not cheap. Apple has reportedly invested many billions of dollars in growing out icloud.

Not through Apple. It's a rental of space on their servers.

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