You need to get RO water and heat it up to the same temp as the tank. Float the fish in the RO water until their temps are even. You are going to want to put some PH 8.3 in the freshwater so that they don't go into PH shock. Make sure the temp and ph are all the same. You will than put the fish in the RO for a few minutes. If the fish shows signs of distress remove it immedietly and make sure you are running an airstone in it. Afterwards dip it out and put it in your tank..
freshwater dips kill ich, flukes and a few other parasites.
Honestly if you are worried I would rather QT the fish than freshwater dip it after a long trip in the mail.