You can give your customers this feature to download invoices after placing orders in Shopify.
Your customers can download the GST invoice from your Shopify store order confirmation Email.
To do this all you need to do is copy and paste the below code into the Email body(HTML). Go to Settings->Notifications->Order Confirmation.
Image for Reference.
Note: If you are not sure how to edit the files, you can contact us on live chat and we will get things done for you.