1 year back, implemented a QR code on our invoices to accept UPI payments. It also fetches the total invoice amount. Just sharing, in case if someone from India finds it helpful.
Have used a service called QR code API to generate QR code in real-time. It is free and have been using it for more than a year now. Have never given an error.
Add an HTML block to your invoice
And then add the following code into the block:
<h6>Scan to Pay</h6>
<img src="https://api.qrserver.com/v1/create-qr-code/?size=100x100&format=png&data=upi%3A%2F%2Fpay%3F%26pa%3D7736060114%40okbizaxis%26pn%3DZARNIK%26tr%3D{{ doc.name }}%26tn%3D{{ doc.name }}%26am%3D{{ doc.outstanding_amount }}%26cu%3DINR">
Replace 7736060114, okbizaxis and ZARNIK.
Try it out here 
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