How to add signature to paid invoice receipt?

Welcome to the Community family, @carolhod.

At this time, there isn't a direct way to add a signature on the invoice template in QuickBooks Online. However, you can utilize the Message field from the template to include the information you need (see screenshot below).

This way, this section will be printed on invoices, and your customer will be able to add his/her signature manually upon paying it.

You can also add the message to your invoice template, so it will automatically appear every time you create invoices.

1. On an Invoice page, click Customize at the bottom.
2. Select Edit current to customized the checked template.
3. Click the Content bar and click the lowest section of the previewed invoice on the right side.
4. Enter Received by: in the message box. 
   

For more tips about customizing invoice templates in QBO, please see this link: Customise invoices, estimates, and sales receipts in QuickBooks Online.

On the other hand, if you're referring to adding your signature, you may use the same field and enter the details that you want. You might also consider checking out for a third-party application that integrates with QBO and supports adding a signature on invoices digitally.

Let me know if you have any other questions. I'll be around to assist you further. Wishing you well!