If you use a currency exchange booth in Bangkok Airport then there are no commissions or service fees applied. However, if you use an ATM then there are bank and ATM operator fees applied. The ATM operator fee is currently 220 baht in Thailand, but subject to possible change. For this reason it makes sense to withdraw the maximum amount at any one time, to reduce the percentage loss of the total transaction amount. The maximum daily withdrawal amount is usually 20,000 baht, so ensure that if you are using an ATM that you take all you can in one transaction. If you encounter this message, ‘invalid transaction please contact your bank’, this means that the ATM does not have enough cash in the drawer, so either reduce the amount, or if there are other ATMs nearby then try again.