SalesTaxOrderValidationStrategy.cs

public class SalesTaxOrderValidationStrategy : IOrderValidationStrategy
{
    public SalesTaxOrderValidationStrategy()
    {
        this.SalesTaxCalculationService = new SalesTaxCalculationService();
    }

    public SalesTaxCalculationService SalesTaxCalculationService { get; set; }

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

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