Negative value in shipping method

-- url of the page with the problem -- : n/a
-- HikaShop version -- : latest
-- Joomla version -- : 3.3.3
-- PHP version -- : 5.6
-- Browser(s) name and version -- : chrome
-- Error-message(debug-mod must be tuned on) -- : n/a

Hi,

We would like to give a discount to customers that pick up their goods themselves.
So we created a shipping method with a negative value. This works in the calculation. (it's a petty that a negative percentage does not work).

In the overview however, the negative shipping cost do not show up although they are calculated. when we try this with a positive value, the value is shown in thje overview.

Is this something that is easy to correct. Or is there an other way to give a discount that depends on shipping method...

Regards, Benny.