It only takes a single stamp. But it does have to be the correct denomination. Currently the rate from the US is to France for a post card is 98 cents.
Just mailed Christmas card, was $1.08
Whenever I have questions on cost for postage I just use this, http://ircalc.usps.gov/ I don't have a direct answer for you, but the next best thing would be to take the card into your local Post Office & pay for the postage there. You can pay for the exact postage rather than havening to deal with spending extra on stamps than you needed to.
* * * * * * Stamp catalog (postage stamps) * * * * * * * * * *
Examples of card catalogs:Stamp catalog (postage stamps)In science:In merchandising:
First class postage was 37 cents for the first ounce in 2005. A post card cost 23 cents.
First class postage was 3 cents for the first ounce in 1940. A post card cost 1 cent.
First class postage was 33 cents for the first ounce in 2000. A post card cost 20 cents.
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First class postage was 8 cents for the first ounce and went to 10 cent in March of 1974. A post card cost 8 cents.
That will depend on where in the world you are posting it from.
If you remove the French stamps, then yes you can. No, that is not allowed. The postage should be the stamps of the country where the card or letter is mailed from. In this case, Belgium wants paid for handling the mail.