You can give your customers the facility to add their business details while they place orders. Your customers can add Company Name and Company GST number while ordering to get GST Credit.
To do this all you need to do is copy Code Snippet available at Settings > Integration in GST APP
See the image for further reference.
To Enable This, Please Copy and paste the code into your cart.liquid file between the '<form>' and '</form> tags. The form field will appear in the Cart page.
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.
Once customer add details here next time if he place order with same email then also on invoice his GST details will come.
Note
Shopify do not allow any customization on Checkout pages so this code snippet can come on view cart page.