So from the above question what I understand is that you are neither importing the goods now exporting the goods you are going to act as a retailer who doesn't keep the stock and fulfills the order so everything would be done outside India.
Therefore in such case I don't think there would be any GST applicable.
I think the company (Shopify) might ask you to have a current account and I think it would be better to have a current account as you are going to receive foreign income so it would be easy for you to bifurcate.
I think later on you will also be required to get some forms signed by CA and get permission from FEMA. Please consult a FEMA expert regarding this.