So it look likes it got something to do with the shipping cost tax calculation:

private static decimal CalculateShippingTax(
            ITaxValue[] taxes,
            decimal shippingTotal,
            decimal shippingCost,
            decimal itemExtendedPrice,
            decimal itemQuantity,
            decimal totalQuantity)
        {
            decimal basePrice = shippingTotal == decimal.Zero
                                    ? itemQuantity / totalQuantity * shippingCost
                                    : itemExtendedPrice / shippingTotal * shippingCost;
            return GetTaxes(taxes, TaxType.ShippingTax, basePrice);
        }

If I modify that function to this:

private static decimal CalculateShippingTax(
            ITaxValue[] taxes,
            decimal shippingTotal,
            decimal shippingCost,
            decimal itemExtendedPrice,
            decimal itemQuantity,
            decimal totalQuantity)
{
        return GetTaxes(taxes, TaxType.ShippingTax, shippingCost);
}

I get the right tax amount for the whole order.

I'm not sure what the original one is doing, why is it using the item price to calculate the tax for the shipping?
And what are the GetPriceExcludingTax do? All prices are excluding tax already?

private static decimal GetPriceExcludingTax(ILineItem item)
{
       return item.PlacedPrice - (item.TryGetDiscountValue(x => x.OrderAmount) + item.TryGetDiscountValue(x => x.EntryAmount)) / item.Quantity;
}

The shipping cost net is 39,2 but this line:

itemExtendedPrice / shippingTotal * shippingCost;

Sets the shipping cost to 35,28. I don't really understands what is happening here?
Then the shipping tax are the calculated on 35,28 and there for we get a diff.

Can someone please enlight me! 

/Kristoffer

UPDATE

The modified function:

private static decimal CalculateShippingTax(
            ITaxValue[] taxes,
            decimal shippingTotal,
            decimal shippingCost,
            decimal itemExtendedPrice,
            decimal itemQuantity,
            decimal totalQuantity)
{
        return GetTaxes(taxes, TaxType.ShippingTax, shippingCost);
}

only works if I have one item in the cart.

/Kristoffer

Hi Kristoffer,

This scenario you point out is probably a bug. I guest the discount you set here is order level discount. 

The reason of using item price in the calculate shipping tax to determine the proportion of difference items in the shipment, to get right amount of shipping cost corresponding.

But the implementation is not correct, when the item extended price is minus order level discount while the shipment subtotal is not. That is a reason why you could not get full of the shipping cost for the tax calculation.

So that is this case, I suggest you using item extended price to item discount price from line item calculator.

/Tuan

Inform, I just file a bug for this case.

/Tuan

Hmm...

I have tried this:

decimal num3 = lineItemCalculator.GetDiscountedPrice(lineItem, currency).Amount * num2;

and this

decimal num3 = lineItemCalculator.GetExtendedPrice(lineItem, currency).Amount * num2;

None of them gives me the right var amount.

Was this what you suggested?

/Kristoffer

Got it to work. I still needed the extended price to calculate correct lineitem tax so here is the workaround:

protected override Money CalculateTaxTotal(IOrderForm orderForm, IMarket market, Currency currency)
{
    decimal amount1 = new decimal();
    foreach (IShipment shipment in orderForm.Shipments.Where(x => x.ShippingAddress != null))
    {
        decimal amount2 = new decimal();
        decimal amount3 = this.shippingCalculator.GetDiscountedShippingAmount(shipment, market, currency).Amount;
        decimal amount4 = this.shippingCalculator.GetShippingItemsTotal(shipment, currency).Amount;
        foreach (ILineItem lineItem in shipment.LineItems)
        {
            int taxCategoryId;
            ITaxValue[] taxValues;

            if (this.TryGetTaxCategoryId(lineItem, out taxCategoryId)
                && this.TryGetTaxValues(market, shipment, taxCategoryId, out taxValues))
            {                
                decimal num3 = lineItemCalculator.GetExtendedPrice(lineItem, currency).Amount;
                decimal shippingLineItemValue = lineItemCalculator.GetDiscountedPrice(lineItem, currency).Amount;
                decimal totalQuantity = shipment.LineItems.Sum(l => l.Quantity);

                amount2 += CalculateShippingTax(
                    taxValues,
                    amount4,
                    amount3,
                    shippingLineItemValue,
                    lineItem.Quantity,
                    totalQuantity);

                amount1 += currency.Round(GetTaxes(taxValues, TaxType.SalesTax, num3));
            }
        }

        amount1 += currency.Round(amount2);
    }

    return new Money(currency.Round(amount1), currency);
}

Use the shippingLineItemValue to calculate the shipping tax and the num3 to calculate the lineitem tax.

/Kristoffer