If you see an invoice for zero dollars, then it's because the fees exceeded the income from the sale. When this happens, we create an additional refund document for the remaining balance. 

The most common fee types that show up on the refund receipts for these invoices are FBA Fulfillment Fees and Sales Commissions, though you may see more than one fee appear on this document.

You may want to check and raise the prices of these items so that you're not losing money on future sales.

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