Whats doing reciepts technically called?

I do receipts for my grandfather every year. He is a trucker and what he does is he gives me all his receipts and i seperate them by month, day and price and once they are all organized i stick them in a folder for him. I get paid about 150 every time. So if i wanted to do that for a living, whats it called? And how do i get started at such a young age? Im 17.