After purchasing the WP Travel Pro plugin or addons, you will get the license key. You need to add this license key and activate license by going to Admin Panel > WP Travel > Settings > License.
For your information, the license will only work for a single domain. You cannot use the same license key for more than one site.
Note: In case of multisite setup, the single license is valid for all sub sites if the license is network activated.
If you have any confusion then please post them in our official support forum