I just left JFK and was asked by the airline to show my 60 visa which I did and even though my return flight was in 90 days there wasn't a problem my visa it stated I can extend for 30 days in Thailand
Cash, use your credit card when you can as long as you have a card that doesn't change extra for international transactions. Also try to have direct payment on your card to pay every month automatically.
You can leave the country and come back without a Visa. You will get a fresh 30 days stay. I believe you can fly out and back often but I think if you it by land it's limited to twice a year?