The Indian postal system is fairly reliable, and stamps can be bought in post offices and various other shops. Airmail letters and postcards take a week to 10 days to reach most destinations from India. Postal delays are caused by holiday rushes (during Diwali and the Christmas season) and strikes.
Airmail letters (weighing 20 grams, or 0.7 ounce) to Australia, New Zealand, United States, Canada, South Africa, or Europe cost Rs. 20; postcards cost Rs. 12. Aerograms to anywhere in the world are Rs. 15. Although your postcards are likely to arrive, they may take a very long time to do so.
There are many options for shipping gifts and other purchases home. Many stores—especially those that cater to tourists—ship items using courier services or other private methods. It's usually reliable and fast, but reconsider shipping anything special that you would absolutely hate to lose. You can send parcels home "surface air lifted" (SAL), a special service provided by the Indian postal department that's cheaper than airmail. A letter or parcel of 250 grams or less costs Rs. 465 to the United States and Rs. 620 to Canada. Each additional 250 grams cost Rs. 101. Different rates apply for other countries.