zero value invoice in GST

We are a Prepaid Wallet Company operating under the Semi-Closed wallet license issued under PPI act. We sell both Prepaid cards & PoS, which both fall under the taxable goods list.  

Now, we are conducting a promotional activity to increase our customer base by issuing the cards at FoC to the Customers. But, they have to pay for loading the card.

In this case, we are not selling our cards. Do we really have to prepare a Zero valued Invoice? If yes, should we mention the tax amount as zero or we should omit the GST Column, at all?

We are a Prepaid Wallet Company operating under the Semi-Closed wallet license issued under PPI act. We sell both Prepaid cards & PoS, which both fall under the taxable goods list.  

Now, we are conducting a promotional activity to increase our customer base by issuing the cards at FoC to the Customers. But, they have to pay for loading the card.

In this case, we are not selling our cards. Do we really have to prepare a Zero valued Invoice? If yes, should we mention the tax amount as zero or we should omit the GST Column, at all?

No one have proper solution for this?