When they say its free, its not really free. They usually give 1% less on exchange rate i.e. 60-65 paisa less per dollar except UMTCL. Check this out

Date: 6 May

Exchange Rate:

Muncha - 64.1025

Thamel - 64.10

MoneyToNepal - 64.10

UnionMoney (UMTCL) - 64.70

Nepal Rastra Bank - 64.75