You can learn how to add your logo to invoice templates.
1. Open Easy Invoice+ from Apps section of your Shopify store or go directly to https://easyinvoice.softify.co/
2. Click “Invoice Templates” from left menu.
3. Edit your primary invoice template.
4. Click “Choose File” from left settings panel of Invoice Template and choose your logo from your computer.
5. Click “Save” button.
Suggested logo size is 250×100 pixels but Easy Invoice+ can also auto resize your logo to your Shipping Label template.