Hi Jenden1,
You can add electronic signatures to payment receipts using QuickBooks Online Mobile App. If you're not yet using it, let's go ahead and install it on your mobile phone. Here's how:
- Open App Store for iPhone or Google Play for Android.
- Search for QuickBooks Online.
- Click on Install.
Once done, open the app and sign in. Select the company file and open the invoice. Click on the X Get signature link to add the signature from the customers. 
If you'd like to add a signature when using a browser, that isn't an available feature. However, you can use the Add attachment section and paste the image of the signature.
I will definitely pass this along to our Product Development team as a a feature request. We would love to hear any ideas from you. Please keep them coming.
If you have any questions, please let us know. We are here to help.