Expect a letter or postcard to take 7–10 days to reach the United States. Postcards mailed internationally cost €1.85; letters the same for up to 20 grams. All post is sent airmail unless otherwise specified.
The Portuguese postal service—the CTT—has a website in English and Portuguese with information such as how to trace mail and the location and hours of countrywide post offices.
You can buy selos (stamps) at correios (post offices) or at kiosks and shops displaying a red "correios–selos" sign. Stamp-vending machines are scattered about Lisbon.