You don't necessarily need to send invoices to your customers in separate emails. Instead, your online store can include links to invoices for your customers to view, download, print or pay online.
There are three different ways you can link to your Sufio invoices:
Add links to invoices in the Order Confirmation emails sent by Shopify.
Put links to invoices on the Shopify customer account page.
Link to an invoice on the last page of the Shopify store checkout.