Convert money to downloads

Hello,
I searched a lot this subject but i didn't find a solution and i am asking you if maybe you can help me!
I want to develop a eshop that will sell ebooks for example.
The steps are:

1) A customer registers at the eshop
2) The customer buys downloads via paypal.. So he will pay 10€ for example and he will have 100 downloads and after the customer will have 100 downloads for downloading ebooks.

My question is if there is any system that makes the above actions!

My thought is after the customer pays via paypal to make a script that converts the amount to downloads...
What do you think.. is it correct or is there better solution?

Thank you very much!

I assume you're looking for your customers to be able to purchase credits?

Yes something like that...
Is there any module that gives this oportunity?

Thank you!!!

possibly something like this?
http://www.opencart.com/index.php?route ... n_id=21706
http://www.opencart.com/index.php?route ... on_id=9244
http://www.opencart.com/index.php?route ... on_id=7201
Ernie

I think none of the above extension is suitable that i am searching for!

I want a module to convert the amount of the customer to downloads..

I think it is very skilled so i have to make it my own...