From WIKIPEDIA:
Postal services in Andorra are unique in that they are not operated by the Principality itself, but by its two larger neighbouring countries, Spain and France. This is a legacy of the centuries of de facto control exercised by those two countries over Andorra. Correos of Spain and La Poste of France operate side by side.
Spanish post boxes are red and French ones are yellow. Both postal administrations issue their own postage stamps for use in
Andorra featuring unique designs, — those of Spain and France are not
valid.
Thank you so much Fabienne for this token during your trip to the Principality of Andorra! =)