Incorrect Grand Total in Magento -

I am using Fixed Product Tax (FPT) which we are calling a "service fee" to charge "service fees" for $ 4 "service fee" for each item in the shopping cart so if someone is 2 If he buys the quantity, he is charged an additional 8 dollars.

This works very well, but when I apply the "promo code" (shopping cart price rule), the total number for each item should be zero (removal of "service fee") It does not recalculate the Grand Total in the shopping cart, even if the discount is visible and it is calculated correctly it seems that the recalculation product is limited to the total, not the FPT amount.

I can not post an image, but this is the car that shows:
Product line item: $ 22 ($ 18 + $ 4 FPT (We call this "service fee") ) Subtotal: $ 22 - Discount (promo): - $ 22
Grand Total: $ 4

Total $ 22 - $ 22 = From $ 0 0 should have been.

I already have System & gt; YES has tried to "apply a rebate on FPT". Configuration & gt; Sales & gt; Tax & gt; Fixed product tax & gt; Apply discount to FPT

The shopping cart price rule uses a "fixed price discount of $ 22" when the product matches the SKU. Any amount of $ 18 (Product base price without FTP) actually affects the same.

I am using Magento Community 1.6.1.0.

If you have decided to include the tax then you should apply a discount on "prices -> gt; ; Taxes can be taxed / excluded from taxes, "you can check it in System- & gt; Configuration- & gt; Sales-> Tax- & gt; Calculations - will be found in the settings.