Add On Shop - How Did You Do That?

It is default shopping cart module. This module is included to aMember by default. You can enable it at
aMember CP -> Configuration -> Setup/Configuration -> Plugins (Modules: cart)
and then apply necessary configuration at
aMember CP -> Shopping Cart

In product setting you will have new options for products and you can set image/descriptions/tags etc here
aMember CP -> Products -> Manage Products -> (edit)

In case you want to highlight Shopping cart button then you can do it with the following CSS in site.css file
http://www.amember.com/docs/Site.css_file

Code:

.am-tabs #menu-cart {
    box-shadow: 0 0 8px #51a7e8;
    border:none;
}

By default module do not give you ability to add additional item to menu (it can only switch default add/renew button with shopping cart button). In case you want to add separate menu item you can do it with the following code in site.php file
http://www.amember.com/docs/Site.php_file

PHP:

Am_Di::getInstance()->hook->add(Am_Event::USER_MENU, function(Am_Event $e) {
    $e->getMenu()->addPage(
        array(
            'id' => 'cart',
            'label' => 'Addons Shop',
            'module' => 'cart',
            'controller' => 'index',
            'order' => 1000)
    );
});

You are welcome!