Using Classic Outlook on Windows for personal email, calendar, and contact management
Hello FoundationsProject,
Welcome to Microsoft Community.
According to your description, seems you wanna increase the storage of your mailbox. Based on this point, the mailbox storage should be related to licenses and subscriptions which is our billing team's support scope, we kindly suggest you refer https://docs.microsoft.com/en-us/microsoft-365/admin/get-help-support?view=o365-worldwide to contact them, and then when you tell your requirement they will help you to choose the proper license and subscription which is the most fit your situation.
Kind Regards,
Tommy