Shopping cart/pages/posts/themes/integration confusion: What is what?

Well, I appear to be doing nothing but asking questions on this forum; however, that’s because I don’t know enough about WordPress or shopping carts to answer such questions for anyone else.

Alas, if anyone out there has questions about writing fiction, I have twenty-five years of experience doing so and would be happy to help you.

As it is, I’m having a hell of a time figuring out the wordpress/shopping carts/storefronts/pages/posts/plugins/integration/themes puzzle. I’ve searched for an answer to some fundamental questions and haven’t found anything so far that explains the differenct in these things well enough for me to understand it. So, if anyone can help me with this or point me to sites that explain the questions below, I would certainly appreciate your help.

I see “plugins” that use “pages/posts” as product links- that “work with” merchant accounts. Then I see “shopping carts” that apparently have “themes” and “integrate” with wordpress. I don’t need a theme; I already have a wordpress site with a theme.

Basically:
1. I already have a wordpress site constructed.
2. I would like to sell downloadable digital products on the site.
3. I want automatic product delivery.
4. I want an external merchant account like Paypal or 2checkout.
5. I want to use a free shopping cart or plugin to do this.
6. I want the products displayed on my site pages, where customers can add them to a cart from my pages and not have to view them on a separate “theme” page that looks alien to my site.

However, I don’t know what kind of “shopping cart” I need to use.

So, the question:
What’s the difference in “plugins” that use pages/posts as product pages—the difference in eshop, wp e-commerce, and ShopperPress, for example—and shopping carts that “integrate” with wordpress using “themes” or whatever? Can I make one of the latter look like the pages on my site? Can I use one of the latter without a “theme” that must be “integrated” with my site?

I just want someone who visits my site to view my books on my site pages, click on an “add to cart” button, check out, and have the produts automatically downloaded to them without me having to do it.

And my offer to help anyone with writing fiction stands.

Thanks