How can I show the customer that I have proof they have viewed an invoice?

Hi user12557,

The status of the invoice (Sent, Viewed and so on) is saved in QuickBooks Online. While there is no option to email or export this as this is not customer-facing information, you could look at taking a screenshot of the status to send to your customer if you choose.

1. Go to the Sales tab, then Invoices.
2. Locate the invoice and select the small dropdown arrow next to the status. This shows the Viewed status and date: 

     3. Alternatively, click into the invoice itself to open it and scroll to the bottom. This should display the               activity with Viewed status as well: 

Use your Snipping Tool, Print Screen function or so on to copy the status as required. 

-Kass