Some info:
1. Most US credit cards charge a 3% surcharge on any foreign transaction. This appears on the monthly bill. An exception is the Capital One Master Card. I use mine in Thailand all the time, at gas stations, larger stores, hospitals, etc. without any charges on my US bill.
2. Anyplace that accepts credit cards has to pay a fee of 1.5% to the credit card bank. As mentioned above, most places will charge you 3-4% for the priviledge of using your card (and make a little extra profit).
3. Most ATM machines in Thailand add the extra surcharge mentioned above. Some do not, but since I never use them I can't tell you which machines to look for. At least the screen lets you know about the fee before processing, and gives you the option to accept or decline.
4. You can get cash on your card without the ATM fee by going into the bank and getting a cash advance at the desk where they do foreign transactions. Takes a bit longer, but saves you money.
5. When you do use the card for purchases, you sometimes are asked if you want to pay in Thai Baht or dollars. Always take the Thai Baht option. Your bank will do the exchange at the current rate, while paying in dollars uses the store's rate - which is usually worse for you.
6. While bank office hours are similar to the US, there are small offices in all the malls and shopping centers that keep the same hour as the mall.
