Customer purchased 10 seats, and now wants more. How do they do this using Checkout?
Customers cannot add additional licenses themselves for the installed App from AppExchange. This cannot be managed in Salesforce Checkout. Instead, Customers need to contact the publisher of the managed package.
The publisher should have installed License management App, linked the package with the License Management App and configured it. The license of a customer can be modified using the Modify License Button on the license record of License Management App. Expiration, Seats and Status can be changed using Modify License Button.
The following links can be referred for more details.