I emailed an invoice to a customer - where can i get proof this was actually done?

Hi there, 

You'll be happy to know that you can absolutely get a record of that email that was sent to your customer through QuickBooks Online! It's actually really easy and I'll show you how. 

To check this, click Customers from the left menu > select the customer in question > locate the invoice and open it. Once you have the invoice page open, click More and click on Audit history. From the Audit history screen you'll be able to see when it was sent out. Give this a try and let me know if this info helps. I'll be on standby.