You will not be charged unless you upgrade to a paid plan.
To install tabs:
Log in to the Shopify App Store.
Go to the Tabs by Station app listing page.
Click Add app.
In your Shopify admin, to authorize the use of the app, click Install app.
After the app is installed, you can view and use it from the Apps page in your Shopify admin.
To show tabs in your shop, you'll first need to activate tabs in a theme.