QuickBooks Online has a feature that lets you know if a customer has viewed the emailed invoice, QHVAC. 

The Invoice activity section in QuickBooks Online tracks the number of times an invoice is viewed. Please know that viewing or opening invoices within QBO will not count as viewed. Only views done by opening the emailed sales form will appear in the said section.

Furthermore, the timestamp includes all views by anyone who receives the invoice, including those added in the CC and BCC fields.

Additionally, QBO has functionality that gives you tools to create personalized and professional-looking sales forms. You can refer to this article to learn how: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Please don't hesitate to reply to this post if you have further questions about viewing invoices in QBO. We're always available to assist you.