Thank you for reaching out to the Community. Sending receipt automatically via email notification to the customer is not available in QuickBooks. Though, you can send it manually. I'll guide you how:
On the left pane click Sales.
Select the Invoices.
Find the paid invoice.
Under the Action column, click the drop-down arrow and choose Send.
Enter the email address under To field.
I understand the importance of this option sending an email notification automatically to the customers when invoices are already paid is much necessary for you and your business. Rest assured, I'll personally send this feedback to our product developers. This way, it might be given a chance to be added to our next products update.
I've added an article to learn more about sending an invoice: Send an invoice.
I suggest visiting our Blog site. This is where we share recent happenings and future developments, such as updates to newly added features. Here's the link: The QuickBooks Blog
If you have other questions, don't hesitate to comment below. Have a great day!