The 2Checkout extension passes in the total in the buyers selected currency and passes in the currency code to override your 2Checkout base pricing currency. This way you can offer pricing in multiple currencies on your website. If you would like to change this behavior, you would need to change the Checkout model to use:
$amount = round($order->getBaseGrandTotal(), 2);
instead of:
$amount = round($order->getGrandTotal(), 2);
This will pull the total from the $order object in your base currency instead of the buyers display currency. Please note however that the 2Checkout PE 102 error is an account related error and is not related to currency.
Please feel free to contact me directly at christensoncraig@gmail.com so that I can help you make the appropriate modifications.