В checkout.php и validation.php заменить везде
Код:
$cart->getOrderTotal(true, 3)

на
Код:
$from_currency = new Currency(intval($cart->id_currency));
$to_currency = new Currency(Currency::getIdByIsoCode('USD'));
$cart->getOrderTotal(true, 3)/$from_currency->conversion_rate*$to_currency->conversion_rate