It is expensive to buy stamps for postcards on Galapagos Islands (same price as in the rest of Ecuador though). For Europe the stamp for one postcard is $2.25 (July 2011), so make sure to bring a lot of cash to the post office if you have many postcards to mail.
And it can take a very long time before the postcards reach their destination. The postcards I mailed in Puerto Ayora didn’t arrive to their destination until almost two months after they were mailed. From Quito it only took about a week. Anyway I would prefer to receive a postcard from Galapagos which is also mailed there, even if it takes a long time.
I took 31 kids to the Galapagos Islands and we had a great time. However, 12 of my group got sick and we suspect it came from the water. Please be extra careful when traveling to this very special place.