Shopping cart help

alright well im pretty sure their isnt a shopping cart tutorial or anything of that sort that covers shopping carts so here it goes. Alright so im using pay pal shopping carts and what not and i was wondering how i would be able to have seperate accounts added to the cart, like say someone buys something and i want to have 20% of that sale go into another seperate account. how is this done? does anyone know? if their is a tutorial on these idea i would greatly appreciate a link to it. also i know some sites have where u can add "shipping charges" to the cart. how is this done also? various sites have where u pick a "plan" of shipping and what company as well and that is added to the cart as an additional cost, how can this be done?