Hello there trucoastal,

It makes total sense that you want to know when your customer has received or read the email that contains their Invoice or Estimate. This keeps you informed as to where they are in the transaction process so that you can better serve them and know when to reach out further. There are a couple of ways to approach this in QuickBooks Online and I want to give you a hand.

While you won't receive an automatic read receipt notification within QuickBooks after your customer has received or opened an Estimate or Invoice, you'll be able to see notice of delivery if you View/Edit the transaction once it's been sent. Beneath the Customer field you'll see Last Delivery with details on the email it was sent to as well as the date and time. When you're selecting a customer from the dropdown, you'll also see the option to Add new which allows you to Connect your Gmail account. This'll allow you to import your contacts for ease of sending transactions, but will also allow you to see the email from the Sent folder in Gmail. There may be an option within Gmail to see if an email has been opened, so it's worth checking into. 

As @Lourdes-Fernandez mentioned, you also have the option to view a Transaction Journal by opening the transaction and using the More option at the bottom. This won't indicate whether or not your customer has viewed the transaction though, only the balance, payment, and accounts being Debited and Credited. 

What you're looking for is the Activities section of an invoice which can be viewed by choosing to View/Edit it within the Action column of the Sales menu. When you scroll to the bottom you'll see Activities, which outline when you sent the invoice and when your customer viewed it.

I know this'll get you the information you need, but I hope you'll reach out again if there's more you need help with. Take care and have a great day!