Changelly doesn’t fix currency rates. This rate you see when creating a transaction is relevant only for this period of time. Please bear in mind that the currency rate remains volatile until the “Exchanging” process is finished.
Because of the volatile nature of cryptocurrency, the rate of any currency can fluctuate every minute until it gets exchanged at a trading platform. This is why the final rate may be slightly higher or lower than expected at the end of a transaction.
Although we never promise to fix currency rates at the beginning of the transaction, we do guarantee that your coins are exchanged at the best available rate of the given moment.