Hi there, philip12,

While we're unable to mark invoices as sent without sending them, you can send them to your email. This way, QuickBooks will mark them as sent and your customer won't receive them via email.

Let me guide you through the steps:

1. Go to Sales from the left panel, then select Invoices.
2. Click the invoice to open it.
   
3. Enter your email address in the Customer email field.
4. Click Save and send.
   
5. Click Send and close in the Send email window to send the invoice.
6. The status of the invoice is now overdue x days (Sent).
   

I've also added this article for additional information: Send an invoice. Then, go to the Check the invoice status section for the description of each status.

Once your customers pay what they owe, you can then record the payment in QuickBooks Online.

Keep me posted if there's anything else you need and I'd be glad to help you out.