This is taken from “Payment terms for services” link found within the Superchargers section of Support.
Pay Per Use
All vehicles ordered after January 15, 2017, and vehicles ordered by January 15, 2017 but built after April 15, 2017, are enabled for Supercharging on a pay per use basis. Any credits for free Supercharging that are given with the purchase of a vehicle, including any annual renewal of credits, expire upon the sale or transfer of the vehicle and are not transferable to any subsequent vehicle owner, or to any other vehicle. Credits for Supercharging expire after a set period of time and do not rollover into future periods.