WCF 4.5 Async Service Operation

WCFNet45AsyncService.cs

public async Task<double> CalculateShippingSubtotalAsync(int productID, string postalCode)
{
    var productServiceProxy = new ProductServiceClient();
    var shippingServiceProxy = new ShippingServiceClient();

    var t = productServiceProxy.GetProductByIDAsync(productID);
    var t2 = shippingServiceProxy.GetShippingCostForPostalCodeAsync(postalCode);

    await Task.WhenAll(t, t2);

    return t.Result.UnitPrice + t2.Result;
}