Some manufacturers give their products 6 digit UPC-E bar codes. Amazon expects UPC-A bar codes. The solution is to run the 6 digit UPC-E through a converter which turns them into UPC-A bar codes.
Google ‘convert UPC-E to UPC-A’ for a converter.
Run your product bar-codes through the converter, stick the modified bar code into your item look up code alias field and use this for the field: Product/StandardProductID/Value.
This topic explains how to pull values from the RMS itemlookupcode alias field.