woocommerce custom plugin need small fix

i have build one plugin which will work only with woocommerce, its job is to add custom fee at the stage of after check out before process payment user need to find city based on city plugin will list of price to deliver and there user need to select any one price and that price need to add as a fee to existing cart subtotal i have done every thing code now i just stuck with adding fee its working when i add fee manually on code and use hook but i need it to call through ajax check below link before bid [login to view URL] can any one help me on this