What category do I put shipping supplies for my eBay business?

You can enter your shipping supplies for your eBay business as a miscellaneous expense with a description of "Shipping Supplies"

Here's how to do this in TurboTax Online:

• Navigate to Federal > Wages & Income > Self-employment income and expenses
• Select or create your eBay business
• Choose  "Add expenses for this work"
• Select  "Other miscellaneous expenses" 
• In the list of expenses, "Other miscellaneous expenses" will now appear.
• Choose the pencil icon to add your description and the amount

Alternatively, you could also consider these "Office Expenses" and skip the step of entering a description.