Change itunes billing currency without changing region/country

How do I set the billing currency for iTunes purchases without changing country? Most iTunes products are priced in USD. I am in Singapore. I noticed that iTunes will charge me in SGD but as iTunes is an offshore billing organization, my credit card company will levy an additional charge on top of the iTunes charge. In addition I have come to realize that the exchange rate used by iTunes is worse than that of my credit card company. iTunes makes money from my purchase as well as from the forex of that trade.

As a consumer I want the right to bill in USD so I can enjoy the more favorable forex rate of my credit card company as well as not have to pay the extra fee for an offshore billing.

How can I change the billing currency of iTunes to USD without changing country? If I change country then my credit card does not work and my purchases gets screwed up.