Can I add VAT number and get deduction after my invoice is generated & payment is collected?

Yes, you can add your VAT/GST details and get a tax refund on the last posted invoice. Add VAT/GST number on the billing page. Please click on 'Incorrect invoice?' link next to the latest posted invoice, and amend the details on the 'VAT/GST' section. Once the details are updated, click on 'Regenerate Invoice'. The regenerated invoice will be emailed to you in few mins. Please note that only the last invoice can be edited within 15 days of it being posted.