This File Management & Digital Store plugin which will help you to control file downloads & sell digital products from your WP site.
WordPress Download Manager is a Files / Documents Management Plugin and Complete e-Commerce Solution for selling digital products.
WordPress Download Manager plugin will help you to manage, track, control file downloads & sell digital products easily from your WordPress site.
Use Password Protection, User Roles Protection to control access to your files.
And simply setup prices when you need to sell the digital item. User can directly download free items and when item has a price user will have to go through cart & checkout.
It has easiest checkout option to give the user better experience in purchasing an item and which always increase the probability of successful completion of an order.
As rather than trying to convince customer to buy something, it would be more helpful to think of a cart optimization as an action to remove barrier to that goal.