Hi @James James,
If you start to run out of storage for your SharePoint sites, you can use the Office 365 Extra File Storage add-on to add more storage in 1 GB increments. You could check the cost in the Microsoft 365 admin center. Before purchase the service, you need to find out what type of billing account you have. And you could purchase the add on by following steps
Buy the Office 365 Extra File Storage add-on
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