You can print your invoices both from your Shopify store admin and Simple Invoice by following the simple steps below.
Shopify Store Admin
Step 1: Click on the Orders.
Step 2: Click on the order you wish to print, click on the More Actions button, and click on the Print with Simple Invoice Option.
The same action can be done inside an order.
Step 1: Open Simple Invoice from the “Apps” section of your Shopify store.
Step 2: Select the order you wish to print in the Orders section, click on Print, and select Invoice.