Gst on export

dear sir

       my client has sold the goods to merchant exporter, such merchant export said charge gst rate igst 0.1% for taxable sales. my doubt was any goods sold to any one igst ( gst rate 5 or 12 or 18 or 28%) was applicable.But such merchant exporter why charges tax invoice gst igst rate 0.1% ?

explain the concept sir