There are two possible reasons for an invoice with zero dollars:
Amazon may have sent out a replacement to the customer.
Or fees exceeded the income from the sale. When this happens, we create an additional document for the fees.
The most common fee types that show up on these documents for these invoices are FBA Fulfillment Fees and Sales Commissions, though you may see more than one fee.
You may want to check and raise the prices of these items so that you're not losing money on future sales.