Discount select list I've checked for a similar question but nothing helped me.
So, i want to add a list in shopping cart with 3 options:
a) No discount
b) 70% discount
c) 65% discount
When the user selects, lets say 70% discount, the discount will be added in total amount and after that is going to select the payment method.

I'm doing this because there is no available module for deposit. The 30% of deposit that i want is like 70% of discount.

Thanks and i hope there is an easy solution that i couldn't find
Try doing this with the Commerce Discount module.  With it, you can create discounts that are either hard-coded dollar amounts or are percentage based.  You'll use the second option.

This module uses Rules to calculate the discount amount based on a product price or the entire order amount.  Read more on the module documentation page: http://drupal.org/node/1706132