Thanks for dropping by here in the Community, @sacheen.
I'd like to let you know that being able to add a digital signature on sales receipts and invoices isn't available in QuickBooks Online.
For now, to get your work done, you may check out for a third-party application that integrates with QuickBooks and supports this functionality.
Please know that our developers are constantly finding and considering new features to be added to the system. This way, we'll be able to improve the product and cope with your business needs.
Lastly, I'd encourage you to visit our QuickBooks Online Blog site regularly, to be updated with our latest news and road-maps.
My doors are always open to help if you have any other concerns with QuickBooks. Feel free to leave a reply below. Have a good one!