Registration of your Autoexpreso sticker is now obligatory and has to be done before Jan. 18 2015.
[EDIT] the deadline was extended to June 19, 2015.
You can do it online at www.autoexpreso.com or in person at one of the DTOP/CESCO offices.
Failing to register can get you a hundred dollar fine.
[EDIT] You can register more than one car in the same account and share the balance between those cars.