Thank you for posting here in the Community, @Nacho978.

Yes, capturing signatures using the QuickBooks Online app is available on all QuickBooks Online Plans including Simple Start. It enables a quick and secure transaction by allowing your costumer to sign an estimate or invoice.

First, download the app from the Google or App store. Then when creating an estimate or invoice, you can get their signature. I'll share with you how, depending on what phone (iOS and Android) you're using.

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.

If you wish to design and personalize your invoices, you may read through this article: Customize sales forms. 

Please get back here if you have additional questions about managing your sales transactions. Have a great day!