Gst- cgst, sgst or igst

Dear Sirs,

A Clearing & Forwarding Agent/ Custom House Agent has only one office at KOLKATA ( West Bengal). It is operating from Kolkata but gives services to various Exporters/ Importers from all over India. All India  Importers and Exporters exports or imports through Custom Stations situated at India-Bangladesh border and obtains services in West Bengal only. Now, my query is what type of GST Service Bill will have to be raised by the Service Provider - Is it SGST/ CGST or IGST.