SalesTaxOrderPurchaseStrategy.cs

public class SalesTaxOrderPurchaseStrategy : IOrderPurchaseStrategy
{
    public SalesTaxOrderPurchaseStrategy()
    {
        this.SalesTaxCalculationService = new SalesTaxCalculationService();
    }

    public SalesTaxCalculationService SalesTaxCalculationService { get; set; }

    public void ValidateOrderSummary(string itemsPrice, ClientPurchaseInfo clientPurchaseInfo)
    {
        States currentState = (States)Enum.Parse(typeof(States), clientPurchaseInfo.ShippingInfo.State);
        decimal currentItemPrice = decimal.Parse(itemsPrice);
        decimal salesTax = this.SalesTaxCalculationService.Calculate(currentItemPrice, currentState, clientPurchaseInfo.ShippingInfo.Zip);

        PlaceOrderPage.Instance.Validate().EstimatedTaxPrice(salesTax.ToString());
    }

    public void ValidateClientPurchaseInfo(ClientPurchaseInfo clientPurchaseInfo)
    {
        if (!clientPurchaseInfo.ShippingInfo.Country.Equals("United States"))
        {
            throw new ArgumentException("If the NoTaxesOrderPurchaseStrategy is used, the country should be set to United States because otherwise no sales tax is going to be applied.");
        }
    }
}