Dont pay through the linked federal bank account upi.
Click on 'View All', then select 'Fi' under 'Other UPI Apps' (it might be behind another 'View All' button). You will be redirected to the app, pay from there.
I have paid bill in partial payments with full bill pay code using hdfc platinum dc and got cashback. Only 20k is allowed in single transaction using dc and bill was above 20k.