The country for the Email ID that you have is of US, so the billing detail & currency has to be of the same country.
The Adobe ID, country & billing address should be same, in this case you can create a separate Email ID (valid) & place order with that. So I guess you should be able to make purchase with USD.
The adobe online are placed through different mode of stores in different part of Globe. In Singapore it is placed via Digital River store, in USA it is through Adobe.com.