Thank you for visiting the Community, @Marilyn.

Let me share some details about tracking a sent invoice to a customer and tell when it was last sent.

Currently, you'll need to manually check and open the emails you've sent to a customer to see whether they have received an invoice. Let me guide you through these steps:

1. On the left navigation panel, click on Sales.
2. Click on Invoices.
3. Select any customer that you want to track.
4. On the right corner, you'll see the pop-up window
5. On the pop-up window, you'll see the invoice activity and the last date sent.

You can also record invoice payments in QuickBooks Online if the customer wants to record payments to connect it to their invoice to balance your account.

If you need any assistance regarding this issue, feel free to comment below. We'd be happy to assist you.