Lets say we have a product that costs 100$ (net price). This product has 3 taxes that must be applied to it: 2 percentage taxes (e.g. 17% and 20%) and one fixed amount tax (e.g. +10$).

My question is - what math is done in this case?

Are the percentage taxes applied on the net price and summed or the first one is calculated and then the second one is done with the price we get after applying the first? When is the fixed amount tax applied? After the other two or the other two are done with net+fixed tax?


Question: Show tooltip on invalid input in edit control
Answer: After further testing, I have decided to put this as an answer so others can clearly spot it. 

The solution is in using EM_SHOWBALLOONTIP and EM_HIDEBALLOONTIP messages. You do not need to create tooltip and associate it to an edit control! Therefore, all I need to do now is simply subclass edit control and everything works :

LRESULT CALLBACK EditSubProc ( HWND hwnd, UINT message, 
WPARAM wParam, LPARAM lParam, 
UINT_PTR uIdSubclass, DWORD_PTR dwRefData )
{
    switch (message)
    {
    case WM_CHAR:
        {
            if( ! isdigit( wParam ) )  // if not a number pop a tooltip!
            {
                EDITBALLOONTIP ebt;

                ebt.cbStruct = sizeof( EDITBALLOONTIP );
                ebt.pszText = L" Tooltip text! ";
                ebt.pszTitle = L" Tooltip title!!! ";
                ebt.ttiIcon = TTI_ERROR_LARGE;    // tooltip icon

                SendMessage(hwnd, EM_SHOWBALLOONTIP, 0, (LPARAM)&amp;ebt);
                return FALSE;
            }
            else
            {
                SendMessage(hwnd, EM_HIDEBALLOONTIP, 0, 0);
                return ::DefSubclassProc( hwnd, message, wParam, lParam );
            }
        }
        break;
    case WM_NCDESTROY:
        ::RemoveWindowSubclass( hwnd, EditSubProc, 0 );
        return DefSubclassProc( hwnd, message, wParam, lParam);
        break;
    }
    return DefSubclassProc( hwnd, message, wParam, lParam);
} 


That's it!

Hopefully this answer will help someone too!