is extra postage required for mailing a gift card
No, you have to use stamps from the country of origin of the mail. In this case you will have to use Italian postage stamps.
21p
44 cents
See: USPO website.
96 cents.
Postage to Europe from the US is 98 cents for a one-ounce letter.
Florida is in the US...
$1
$1.20 is the rate in December, 2015.
It depends on which country you are posting from and to which country.
Unless your card is larger than normal, it shouldn't cost any more than a first-class postage stamp to mail a Christmas card from the US to the UK.