Create a new account to sign in to your existing account (https://www.printstop.co.in/user_login_popup.php)
Go to My Account -> Shipping Addresses (https://www.printstop.co.in/user_address.php)
Edit the address where you want to add the GSTIN. Ensure that you put the correct registered company name, address, phone number and GSTIN. Save the address.
Now when you want to order a product, ensure that this address with the GSTIN is selected as the BILLING ADDRESS in your Shopping Cart.