How do I change discount amount in Sales Invoice using GP Web Services?

I'll give you the trade discount code because that can be a little tricky because it can be a money amount or percent.  I don't know what you mean by drop ship fee and the freight charge is just like setting any other amount.  

             //here is how to set the trade discount amount

           //TradeDiscountAmount = new MoneyAmount();

           //TradeDiscountAmount.Value = 1.00M;

           //TradeDiscountAmount.DecimalDigits = 2;

           //TradeDiscountAmount.Currency = "USD";

           //TradeDiscount.Item = (MoneyAmount)TradeDiscountAmount;

           //here is how to set the trade discount percent

           TradeDiscountPercent = new Percent();

           TradeDiscountPercent.Value = .01M;

           TradeDiscount.Item = (Percent)TradeDiscountPercent;

           salesInvoice.TradeDiscount = TradeDiscount;