In Portugal, the best place to buy stamps is at the post office (CTT or Correios). Most tourist shops ask if you want to buy stamps, but the answer is always NO. The stamps are sold at the prices they want (in Portugal the postage price is regularized) and the stamps may not always be valid (the stamps lose their validity after x years from the date of issue). Usually the stamps that are sold are ugly and self-adhesive (at post office), but if you ask, there are usually traditional gum stamps.
The current price is €0.95 for Europe (includes all European Union countries and regions, United Kingdom, Andorra, Russia, Belarus, Iceland, Bosnia and Herzegovina, Kosovo, Serbia, Macedonia, Switzerland, Turkey, Moldova, Ukraine , Montenegro and Norway) and €1.05 for the rest of the world and the United States (standard mail and up to 20 grams).
The best place to deliver mail is to the mailboxes next to post offices. They are usually identified, but if they are not (Red is for national and international regular mail and Blue is for urgent and international mail). In some branches there is a specific box for International Mail.