SOLVED - how to add custom field with calculation on checkout

-- HikaShop version -- : 3.2.1
-- Joomla version -- : 3.4.5
-- PHP version -- : 5.6
-- Error-message(debug-mod must be tuned on) -- : none

hi

how can i add a custom field for change on checkout?

example:

i have created a custom field named "cash on hand". if the shopper chooses to pay on delivery, store owner would like to know the cash on hand he has for the change.

so on checkout, if shopper needs to pay .5usd, he will fill in the custom field "cash on hand". lets say he has 1usd. the custom field will compute and display on the front end change say which is .5usd

the custom field "cash on hand" aims to help the store owner prepare exact change

thanks