How to edit PayPal API (accept payment)?

I want to edit this PayPal API. I want to add a field name "Tax" under "Item total" and get a 2.5% of total amount and display it on the new field name "Tax"

Please refer to this (this is what i want) process (just scroll down in the bottom and make a sample payment): http://ift.tt/1XhMENm then after you submit you'll see the Shipping and handling field name

via Chebli Mohamed