UPI payment has enabled tracking actual sales data and helped with tax collection. This is an intangible benefit which is likely to go away with the introduction of the fee.

The government being able to monitor and control how every individual spends money is a double-edged sword though

How would a fee preclude tracking sales data and collecting taxes?

Small merchant will stop taking UPI

Right, this has second and third order effects. I am sure this must have been brought up during the discussions and overruled for whatever reason. The bureaucrats and politicos involved are not stupid. But there is some game being played here.