nasty problems with taxes and coupons

-- HikaShop version -- : 2.6.4
-- Joomla version -- : 3.6.4
-- PHP version -- : 5.6.26
-- Browser(s) name and version -- : all

Dear Hika Shop,
can you please assist me in finding a solution in coupon tax calculating.
The discount value of the coupon is always calculated on the gross price independently if I set "Product tax category" or not in the Coupon configuration.
In my case I'm showing all net prices, and I would like to show also coupon discount amount as net.
This problem create errors also on the tax calculating.

# Here what it is shown:
Subtotal 143,00 €
Coupon -59,49 € *
IVA 4 % 3,34 € *
Total 89,23 €
(the sum doesn't match 143.00-59.49+3.34=86.85)

# Here what should correctly be:
Subtotal 143,00 €
Coupon -57,20 €
IVA 4 % 3,43 €
Total 89,23 €


You can try it yourself in this test website.
www.iandp.it/test/

Product to choose: "Grand Cru Musignano - Can 5 L"
Coupon to add: "JH40SP" (this is a 40% coupon)

If you need, I can send you in private the admin login credentials.

Thanks for your support
Greg

Hi Nicolas,
I've tried all the four combination having always the same result.
here attached an example image.

Thanks
Greg

Hi Philippe,
Notthing strange on my side, here attacched I post you the process I can do.
Can't you see the same?

www.iandp.it/test/en/boutique-en

Thanks
Greg

Hi Philippe,
I've tried your solution, to reach the correct discount and VAT I had to use a percentage of 38.462% (40% / 1.04 VAT). But still the Total doesn't match. (please see attachment)
This happen either if i set Discount TAX calculation Before or After taxes.

Sorry for this, but I think that the discount percentage shouldn't be modify, this because the discount should be applied to the Subtotal indipendently if it is Gross or Net. It is a percentage so this shouldn't create any arithmetical problem.

Thanks
Greg

Hi Philippe,
I'm sorry you get it wrong, this was not my intent.
The one you exposed is the actual method we are using in the live environment. We have also modified the VAT description (di cui IVA 4%) = (of witch VAT 4%). But this is not what the client is asking.

I don't know in other countries but in Italy commercial invoice are all done as follow:
List of Products (excluded VAT)
Subtotal (excluded VAT)
Discount (on Subtotal)
VAT 4% (on sum of all 4% products * (1-discount %))
VAT 22% (on sum of all 22% products * (1-discount %))
Total (sum Subtotal-Discount+VAT 4%+VAT 22%)

So this look normal to me, and this is way I was asking for a check on this.

Now I understand there is no way to reach that goal. I've take a quick view of your code and I understand it is too much structurated to make a simple override for this situation.

Thanks for your time
Greg