I experienced ordering medicines over Whatsapp for the first time from the local store in Mumbai a few weeks ago. Discussing order items and them checking drawers was a PITA, so I just called them to get it done. There often is no inventory management system - it's a manual check.
It was easy to tell that the person at the store didn't quite grasp how things worked around online payment, just that they knew how to follow instructions someone had given them.
1. Generate physical invoice manually
2. Send customer a pic of the invoice over Whatsapp to reconfirm order items and total amount
3. Send customer a pic of the physical panel w/ QR code from payment provider over Whatsapp
4. Wait for SMS from bank account confirming amount paid
5. Dispatch order
Not quite there in terms of sending UPI "collect requests" or digital invoices, but it works! Receiving a picture of a QR code was a bit odd but you can easily get around that.
This space is definitely exciting, and you can already see the impact digital payments are having on small biz. Good luck, and change the world!