Sorry, was on mobile typing that, and turns out I was wrong.
The UPI over credit card is only for Peer to Merchant transactions.
To be a merchant (As against a peer merchant), you must have a month over month inflow of atleast 50,000 INR, for 3 months after being onboarded as a peer-merchant. [1]
I think you were talking about registering as a peer-merchant, not a "proper" merchant! MDR etc cannot be charged on peer-merchants[1], and are essentially peers, but with restricted purpose codes that they can accept money for.
[1] https://www.npci.org.in/PDF/npci/upi/circular/2019/Circular%...