Hi @milo last,
Per my research, you can purchase an unlimited amount of more SharePoint storage in SharePoint admin center. And there is only a 25 TB for max storage per site. You could refer to following document
Add more SharePoint storage to your subscription
If you keep getting the error during your purchase, You can reach Microsoft store support via the link to guide you through the purchase:
https://support.microsoft.com/account-billing/4f615f2a-6bbd-fd69-6695-ae213d63eef0
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