I want to make sure you'll get the help you need by routing you to the right support, prashant2.

I'd love to sort this out for you, however, it'd be best to contact SAAS Customer Support since it's a third-party application. From there, they'll be able to process your inquiry about importing customer's invoices automatically from your SAAS application into your account. 

Any concerns about syncing or integrating the third-party software into your account are best handled by their technical support. If you already added the app in QuickBooks Online (QBO), here's how to reach them:

1. Go to the Apps menu.
2. Click My Apps. 
3. Choose your SAAS app to contact their technical support. 

Visit our Manage Customers and Income page for more insights about managing your customer transactions.

This will take you in the right direction today, but please don't hesitate to get back to me if you have any follow-up concerns. I'm always here to help. Have a great day ahead. 

@prashant2 

You may need a billing app and integrate it with your QBO.