Hello there, @sal74.

I am here to provide insights into why your customer can view the invoice you haven't sent yet in QuickBooks Online (QBO).

Kindly check your records for any recurring invoice transactions that were automatically dispatched to your customer. The "Automatically Send Emails" option within the recurring invoice feature may have enabled them to access the specific invoice.

 If you do not have any recurring invoice set up, consider reviewing your audit log to verify that the invoice wasn't created.

Moreover, I'm adding this helpful article as your reference in recording your invoice payments in QuickBooks Online: Record invoice payments in QuickBooks Online.

Please don't hesitate to return to this post if you have other concerns regarding your invoice in QuickBooks Online.