WooThemes doesn't just create the WooCommerce plugin. They also create standalone plugins and hundreds of extensions that add extra functionality to WooCommerce. The beauty of this system is that WooCommerce is very easy to use because users only add extra complexity when they need it. If you only need simple shipping options, you don't ever have to see the complex shipping settings.
On the WooThemes website, you may browse for WooCommerce extensions, purchase them, and download and install them on your site. WooThemes has made the whole process very easy to maintain. They have built an updater similar to the one in WordPress, which, once configured, will allow a user to update a plugin with one click instead of having to through the whole plugin upload process again.
Make sure you have the necessary permissions to install plugins on your WordPress site. You also need to have a WooThemes product. There are several free WooThemes products including Pay with Amazon which you can find at http://www.woothemes.com/products/pay-with-amazon/.
There are two parts to this recipe. The first part is installing the plugin and the second step is adding your license for future updates. Follow these steps:
Log in to http://www.woothemes.com.
Click on the Downloads menu:
Find the product you wish to download and click on the Download link for the product. You will see that you get a ZIP file.
On your WordPress site, go the Plugins menu and click on Add New.
Click on Upload Plugin.
Select the file you just downloaded and click on the Install Now button.
After the plugin has finished installing, click on the Activate Plugin link.
You now have WooCommerce as well as a WooCommerce extension activated on your site. They're both functioning and will continue to function. You will, however, want to perform a few more steps to make sure it's easy to update your extensions:
Once you have an extension activated on your site, you'll see a link in the WordPress admin: Install the WooThemes Updater plugin. Click on that link:
The updater will be installed automatically.
Once it is installed, you need to activate the updater.
After activation, you'll see a new link in the WordPress admin: activate your product licenses. Click that link to go straight to the page where you can enter your licenses. You could also navigate to that page manually by going to Dashboard | WooThemes Helper from the menu.
Keep your WordPress site open in one tab and log back in to your WooThemes account in another browser tab.
On the WooThemes browser tab, go to My Licenses and you'll see a list of your products with a license key under the heading KEY:
Copy the key, go back to your WordPress site, and enter it in the Licenses field.
Click on the Activate Products button at the bottom of the page. The activation process can take a few seconds to complete.
If you've successfully put in your key, you should see a message at the top of the screen saying so.
A plugin that's not hosted on WordPress.org can't update without someone manually reuploading it. The WooThemes updater was built to make this process easier so you can press the update button and have your website do all the heavy lifting.
Note
Some websites sell official WooCommerce plugins without a license key. These sales aren't licensed and you won't be getting updates, bug fixes, or access to the support desk. With a regular website, it's important to stay up to date. However, with e-commerce, it's even more important since you'll be handling very sensitive payment information. That's why I wouldn't ever recommend using a plugin that can't update.
Refer to the preceding recipe, Installing WooCommerce plugins, for more details. There are several screenshots in that recipe about the general process of uploading a WordPress plugin.