How to download files to WordPress? Help!

I am trying to sell Photoshop actions on my site.
However, I don’t know how to do this. How do I upload digital files onto my WordPress that will be able to be downloaded once the payment is made?

If you don’t know what a Photoshop action is, I will briefly tell you. It is pretty much a set of steps recorded in Photoshop that people can sell or buy. It’s just a download that people can buy if they want to Photoshop/edit their photos a certain way.

And you see, I’m trying to sell actions. I know a lot of people who somehow have their WordPress site set up where you:

1.) See the action(s) you want.
2.) Click “Add to cart.”
3.) Pay for it via Paypal or credit
4.) Have an automatic email sent to the customer with the downloadable action links.

The question is…how do I upload downloadable files to my WordPress so it can be downloaded by someone once they pay for it? Where do I put it? How do I make it unable to be downloaded until its payed for? How do I upload, create, store, and later download?! This is really frustrating me..

I’ve also tried FileVilla or something, but I don’t know how to use it, and don’t know if that even has to do with what I need.

So if ANYONE kind enough to help figure this out, please help!