Hello Srinisha !!

If you already registered in the earlier round(s) of counselling ,you must have you paid at that time . Therefore, you do not  have to pay again after registering into mop up round .

To know more this please visit the following -
https://www.google.com/amp/s/medicine.careers360.com/articles/neet-counselling/amp


All the best. 

Hope this helps .