My mistake - I finally searched help in the company file and found the following, which at least lets me know if I sent the invoice...
You can find out whether you already sent a particular form, for example, an invoice to a customer, by checking its transaction history.

If you use Outlook, Outlook Express, Thunderbird, or web mail, you can find the emails you sent in your Sent folder.

To do this task

Open the form you want to send.

At the top of the form, click the Reports tab.

Click Transaction History.

Check the information shown in the upper half of the Transaction History window.

If you sent the form, the date you sent it appears in the Sent Date field. If you sent it more than once, only the most recent send date appears.

The Send Method field shows how you sent it (for example, "Email").

If you selected the Email Later checkbox on the form but haven't sent it yet, "Pending" appears in the Sent Date field. To send the form, along with the others marked as "Email Later," choose File > Send Forms, and then click Send Now.

****If you see a message that no payment or transaction has occurred, you didn't send the email, nor did you select the Email Later checkbox. To send the form now, click the Email icon on the Main tab at the top of the form, and then click Send Now.