Let's get your customer's signature added to their invoices, Abuster.

It could be that we've missed a step, or the signature wasn't successfully saved in the process. 

Let's follow these steps and make sure not to skip a single step. Since capturing signatures can be done using the QuickBooks Online app, just follow the details below depending on your mobile device.

iOS

1. From the Menu ☰, select Estimates or Invoices.
2. Select an existing transaction, or create a new one.
3. Add any necessary information.
4. Select Save.
5. Select Get Signature.
6. Have your customer sign with their finger.
7. Select Done.

Android

1. From the Menu ☰, select Estimates or Invoices.
2. Select an existing transaction, or create a new one.
3. Add any necessary information.
4. Select Done.
5. Select Get Signature.
6. Have your customer sign with their finger.
7. Select Done.

You can then view the signature when you preview the form.

Also, I have tried sending a sample invoice to my personal email, and the signature successfully appears. See the sample screenshot below.

If it persists, you can follow these troubleshooting steps:

• Restart your phone and launch again the app.
• Update your phone's operating system.
• Uninstall/Reinstall the app.

Still,  you can check this shared article for more additional details about invoices: Record invoice payments in QuickBooks Online.

Visit again if you need anything else. Take care and have a good one, Abuster.