Current cost to mail a modestly-sized, rectangular birthday card to the UK is $1.10 (April 2013).
Note that as of January, 2013, the Global Forever stamp can be used to mail a postcard or 1 oz letter-size mailpiece anywhere in the world, regardless of when the stamp was purchased and how prices change in the future. To help you distinguish the difference between the domestic and international forever stamps, the international version will always be round and will always bear the words "Global Forever." Right now, April 2013, these cost $1.10 each.
Also note that the postage goes up to $1.30 if the envelope is square or an "odd" shape. This web page might be helpful:
http://ircalc.usps.com/?country=10143
That will depend which country you are in, the destination of the envelope, and by which method you are mailing it.
A stamp for a standard envelope (birthday card ect. Costs €0.82 from Ireland to austrailia.
"No, the cost of mailing a letter doesn't depend on the shape of the envelope but how much weight is actually inside the envelope. It's based on the weight."
As long as the envelope doesn't weigh too much (If you're just mailing paper, then it probably doesn't) then no it doesn't cost more.
is extra postage required for mailing a gift card
Depends from which country to which country.
1 stamp (whatever the current price of a stamp is)
44 cents
It depends from which country to which country.
A birthday card in the mail will cost 49 cents. A post card will cost 34 cents and can be no larger than 4" by 6".
Depending on where one is mailing a postcard from to France, the price on a stamp will vary in cost. If one is mailing the postcard from a location close to France the price is not going to be as high as it owuld be if one were to mail the postcard from across the globe.
That depends entirely on which country you are in and where you are mailing it to