I have an ordercalculator that checks to see if a product has an orderline property that states that it is a donation. There is a global variable that stores the amount of the donation (which is set on the front-end by the user). The code is as follows:

using System.Collections.Generic;

using TeaCommerce.Api.Common;

using TeaCommerce.Api.Dependency;

using TeaCommerce.Api.InformationExtractors;

using TeaCommerce.Api.Models;

using TeaCommerce.Api.PriceCalculators;

using TeaCommerce.Api.Services;

namespace Tivilon.reachoutyouthsolutions.Extensions.TeaCommerce {

  [SuppressDependency( "TeaCommerce.Api.PriceCalculators.IOrderCalculator", "TeaCommerce.Api" )]

  public class TeaCommerceOrderCalculator : OrderCalculator {

    //used to store the donationAmount in CalculatreOrderLineUnitPrice since we will need it in CalculateOrderTotalPrice

    private decimal _donationAmount;

    public TeaCommerceOrderCalculator( IVatGroupService vatGroupService, ICurrencyService currencyService, IShippingMethodService shippingMethodService, IPaymentMethodService paymentMethodService, 

      IShippingCalculator shippingCalculator, IPaymentCalculator paymentCalculator, IProductInformationExtractor productInformationExtractor )

      : base( vatGroupService, currencyService, shippingMethodService, paymentMethodService, shippingCalculator, paymentCalculator, productInformationExtractor ) {

        //Doesn't extend the constructor of OrderCalculator      

    }

    protected override decimal CalculateOrderLineUnitPrice( OriginalUnitPrice originalUnitPrice, OrderLine orderLine, Currency currency, Order order ) {

      decimal? result = null;

      //Check whether or not the orderline is a donation.

      string isDonation = orderLine.Properties[ "donation" ];

      if ( !string.IsNullOrEmpty( isDonation ) && isDonation == "1" ) {

        decimal? donationAmount = orderLine.Properties[ "donationAmount" ].ParseToDecimal();

        if ( donationAmount.HasValue ) {

          result = donationAmount.Value;

          _donationAmount = donationAmount.Value;

        }

      }

      if ( !result.HasValue ) {

        //If we get here then this orderline does not meet the requirenments of being a donation. Let OrderCalculator set the unit price, 

        result = base.CalculateOrderLineUnitPrice( originalUnitPrice, orderLine, currency, order );

      }

      return result.Value;

    }

    protected override Price CalculateOrderSubtotalPrice( IEnumerable orderLinesBundleTotalPrices, Currency currency, Order order ) {

      Price result = base.CalculateOrderSubtotalPrice( orderLinesBundleTotalPrices, currency, order );

      CustomProperty discount = order.Properties.Get( "discountValue" );

      CustomProperty discountIsPercent = order.Properties.Get( "discountValueIsPercent" );

      //if there is a discount code and a value indicating whether or not it is a percent then we need to create a new price object to return and add an order property.

      //Check for ServerSideOnly to verify that these where set on the server and not via JavaScript.

      if ( discount != null && discount.ServerSideOnly && discountIsPercent != null && discountIsPercent.ServerSideOnly ) {

        decimal? discountValue = discount.Value.ParseToDecimal();

        if ( discountValue.HasValue ) {

          //We need to apply a discount to the order total, but not the part which is the discount.

          decimal valueWithoutDonation = result.Value - _donationAmount;

          decimal discountAmount;

          if ( discountIsPercent.Value == "1" && discountValue.Value >= 0 && discountValue.Value <= 100 ) {

            //bring the discount from it's [0-100] format to the [0-1] format

            discountValue = discountValue / 100;

            discountAmount = valueWithoutDonation * discountValue.Value;

            valueWithoutDonation = valueWithoutDonation * ( 1 - discountValue.Value );

          } else {

            if ( discountValue.Value < valueWithoutDonation ) {

              discountAmount = discountValue.Value;

              valueWithoutDonation = valueWithoutDonation - discountValue.Value;

            } else {

              discountAmount = valueWithoutDonation;

              valueWithoutDonation = 0;

            }

          }

          order.Properties.AddOrUpdate( "discountAmount", discountAmount.ToString() );

          decimal newVat = result.Vat - discountAmount*order.VatRate.Value;

          result = new Price( valueWithoutDonation + _donationAmount, newVat , valueWithoutDonation + _donationAmount + newVat ,CurrencyService.Get( order.StoreId, result.CurrencyId ) );

        }

      }

      return result;

    }

  }

}