Andorra is a tiny country (408 sqkm, 79.000 inh), tucked between France and Catalonia (by now, under Spanish occupation), whose heads of states are the President of France and the Catalan Bishop of la Seu d'Urgell (just 20 km from Andorra). This comes from an agreement of 1276.
As traditionaly, the country was very poor and the taxation level very low, it hadn't money to build a postal system. This was not a problem until early XXth century. On the Rome 1906 UPU congress, the Spanish PO took the responsibility of providing a postal service to Andorra.
In 1930 the dual Spanish/French PO system was stablished. The Spanish PO issued its first Andorra stamps in 1928 and the French one did in 1931.Both were overprinted Spanish or French definitives.
As far as I know, the free domestic mail, was one of the services provided by both PO. Anyway, this is not so expenssive due to the country's size, and now it should be near zero. Anyway, the yield produced by the philatelic sales of both PO should be more than enough to cover for the Andorra domestic mail.
As Andorra is a turistic and shopping resort for people from Catalonia, Spain and Southern France, a lot of postcards were sent from there. On the time of old currencies (pesseta and franc) the normal thing was to buy stamps on your currency (pesseta on the Spanish PO, franc on the French PO).
I've got a lot of postcards circulated from Andorra to my city, Barcelona (the big city closest to Andorra) and all of them bear Spanish PO stamps. I guess, the opposite should happen to postcards sent to Tolouse, in Sothern France.
There's some information abour Andorra Postage and stamps, mainly written in Catalan (Andorra's national language), French or Spanish, languages on which I have a good command, being the first my own one.
