The following are links to Post Offices at which you can order stamps online using a credit card. All sites are secure unless noted. If you are interested in finding contact information for post offices in countries that do not offer online sales, try Graham Rhind's "Links to Post Office Pages". Also, this Currency Converter may come in handy.
Aland Island (part of Finland)
Australia Post Stamp Shop (includes Christmas Island, Australian Antarctic Territory, and Cocos (Keeling) Islands)
Faroe Islands (Post verk Foroya)
Finland Post StampStore
Guernsey Post Office (includes Guernseyand Alderney)
India Post (standing orders only)
Isle of Man Post Office - Stamp Center
Namibia (Site not secure!)
New Zealand Poste Stamp Centre (includes Ross Dependency, Vanuata, Tonga, and Pitcairn Island)
Switzerland (Swiss Post)
Taipei Stamp Store (Stamps from China and Taiwan - site does not work with Netscape Communicator!)
Gibraltar Philatelic Bureau Ltd.
The Postal Store of the United States
A page with links to post office web sites for many countries: Click here!