If you sell, or have sold, packages externally to a client, you can assign a package to a customer through the "Customers" page. If you have packages set up in your FlexBooker account then you will see an icon in each customer row that looks like a folder:
If you click on this, then the package popup will appear:
You can add any package (or even multiple packages to the same customer) by clicking on the "Add Now" button.
Any packages assigned to the customer, including both those that you have manually assigned and those that the customer has purchased themselves, will appear in this page and will show you when each one was purchased and how many sessions or minutes remain in each purchased package: