The licensing and billing terms for an app are provided by the vendor who sells the app. The procedures for agreeing to license terms, assigning seats, renewing a license, and providing payment for the license may vary from vendor to vendor, in general they follow this format:

• On the app's Marketplace page, you'll see the licensing options available for that app listed under the Add it now button.
• When you select an app to purchase and enter your domain name, you'll see a new page hosted by the vendor where you can select the licensing option you want and see the total amount of your order.
• When you return to the Marketplace, the Pricing & Terms of Service page will display your order in detail and provide links to both the Google Marketplace terms of service and the vendor's terms of service.
• You'll pay for your order using Google Checkout, then click the I agree, pay $xx now button to proceed.
• On the Setup page, you'll see what data types the app will access. You'll need to grant data access to use the app. You should only install an app if you trust the app's creator. You can cancel out of the purchase at this point if you change your mind.
• Finally, you'll enable the app and see the app's settings page in your Google Admin console.