Is E-Invoicing system need buy digital certificate

You’ll only need Digital Certificate if you’re submitting e-invoices via a third party app, accts3. 

You can consider using third party solutions to integrate with your QuickBooks Online (QBO). Visit the Intuit App Center and enter a keyword in the search bar to explore the options, or alternatively, follow these steps.

Here's how:

1. On the left panel, click Apps.
2. Go to the Find Apps tab.
3. Use the search bar to look for an app.

Also, I'm adding these links for future reference in managing progress invoices and payments in QuickBooks:

• Record invoice payments in QuickBooks Online
• Customise invoices, estimates, and sales receipts 

If you require further assistance with managing your invoices, please tag my name in the comment section. We’re here to help, accts3.