When buying the GPS Tracker, you have 3-year data plan included. This leaves you plenty of time to judge if the product fits your needs.
After this period is over, you will be notified in the app and by email that it is time to renew your subscription, for a cost starting at 9.90€ per year (for a 3 years renewal)*. Just follow the instructions to renew. You will have the option to buy a several-year data-plan for an interesting price.
*Except in Switzerland where connectivity costs are higher.