I want to give my used iPhone X to a friend in Bangkok but my trip has been delayed so I would like to know the best way  (most cost efficient) to send the phone to Bangkok from USA . 

It’s extremely likely that your friend will be asked to pay tax and VAT on the new price of that phone. If it’s hand carried that will be avoided, but any safe carrier will declare the phone to customs, whatever price you declare will be ignored by customs, they will go by the highest price they can find and assess import costs on that.