Thanks for coming to the Community for help, @aljenlove.

I can walk you through how to mark an invoice as sent in QuickBooks Online.

You would need to send the invoice to yourself in order to change the status to Sent. This is the workaround we can use to get the outcome you wanted.

Here’s how:

1.    Go to Sales from the left menu and select Customers.
2.    Choose the customer by clicking it. 
3.    Locate and open the invoice.
4.    Enter your email address on the right side of the customer name.
5.    Click Save and send.
6.    Hit Save and close.

Once done, you’ll find the invoice marked as Sent. You may check out this article for your reference: Send an invoice.

If you wish to have this option added in future updates, I recommend sending product suggestion to our Developers. Simply click the Gear icon at the top and select Feedback. 

We always take your ideas into consideration when looking for product improvements. You may also send your feedback by going to the following link: QuickBooks Online Feature Requests. I’ll make sure to do the same on my end. 

That should do it. Let me know what else I can do to help you succeed today. I’ll be more than happy to answer should you have additional questions. Have a good one!