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Categorize used for inventory supplies in QuickBooks Self-Employed is unavailable. You can read this article to learn more about the categorize used mandated by the IRS for self-employed individuals.
I recommend contacting your accountant for guidance in choosing the category of your supplies. You can find one through this link: Find an Accountant or Bookkeeper.
Here's an article you can read for help with QuickBooks related tasks: QuickBooks Self-Employed Overview.
Keep me posted in the comments if you have any other questions. Thanks!