Currently SAF-T isn't yet available in QuickBooks Online.
I can see how beneficial to you and your company to have this option. For now, I suggest submitting feedback directly to our product engineers. This way, they can look into this option further and consider adding it in the future update.
Here's how to send feedback:
Sign in to your QuickBooks Online account.
Click the Gear icon, then select Feedback.
Enter your product suggestions and click Next to submit feedback.
I recommend checking this article for some tutorials about the uses of different features in QuickBooks Online: QuickBooks Tutorials.
Please continue to reach me here in the Community if you need more assistance managing your invoice template. I'll be sure to get back to you.