By mail in Saudi Arabia
Saudi Post has a good network of post offices around the country, but offices are closed Thursday and Friday. Stamps for postcards to anywhere in the world cost SR4. The bigger problem is actually finding postcards, as the mutawwa periodically crack down on the celebration of non-Islamic holidays like Valentine's Day, Christmas or even birthdays, causing all cards of any sort to disappear from bookstores! Your best bet is thus gift shops in major hotels. Mail coming in to the country from overseas is notoriously unreliable. Stories abound of thing arriving months after they were sent or never arriving at all. There are branches of DHL and FedEx operating throughout the kingdom so a good rule of thumb is to have anything important sent through those channels.
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The Most Frequently Asked Travel Questions about Saudi Arabia