Deemed export under GST

Dear Sir,

We have to supply  taxable as well as exempted  goods to  one of our customer,  who in turn will  export the same to his foreign buyers. We request you to  clarify the following doubts.

1.  Can we treat this  as  deemed export and claim back  the GST for the taxable goods as per our supply invoice ? 

2.  If we are eligible  for the  refund, then what is the proceedure involved ?  What are the changes we have to make in our supply invoice with regard to this supply and what are the documents to be enclosed ?

3.  Finally,  where shall we mention this invoice amount in GSTR 1, whether in column No. 4A, 4B........( along with our domestic supplies )   or  in  Column No. 6A ( for export Invoices ) ?

Kindly advice us.

Thank you.