Hello, 

As you have attended first and second round of counseling, but you didnt get your seat, so definitely you can participate in mop up round without paying any sort of fees as you have already paid once and didnt get your pg seat. So you can directly get your pg seat with fresh registration for mop up round .

Also chevk this link out; 

https://medicine.careers360.com/articles/neet-pg-counselling

Good luck