Get cart class in Ajax function

Hi,

You're not loading HikaShop in your code. It's possible it will work if you have a system plugin of HikaShop loading HikaShop before your plugin is triggered by com_ajax, but it's only luck.
You should yourself load HikaShop before trying to use HikaShop with such code:

if(!@include_once(rtrim(JPATH_ADMINISTRATOR,DS).DS.'components'.DS.'com_hikashop'.DS.'helpers'.DS.'helper.php')){ return false; }