Best Way to Add Fields to Address during Checkout

-- HikaShop version -- : 2.5.0
-- Joomla version -- : 3.1
-- PHP version -- : 5.4
-- Browser(s) name and version -- : Chrome
-- Error-message(debug-mod must be tuned on) -- : How to add custom fields on the "Address" part of the checkout process?

Hi,
I want the customers to be able to make some selections before they receive their shipping rates during checkout. So somewhere where they enter or verify their address I want to add some options. For example options like, 'call before delivery', lift gate, etc etc.
What's the best way to do this via a shipping plugin? What functions do I need to call etc.
Thanks!