Postage stamps, presumably!
In the UK the Royal Mail class a 9" x 12" as a large letter (max 750 grams), postal rates as at June 2011 are 58p second class or 75p first class for UK delivery.
The related link has all the Royal Mail rates etc.
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The answer will depend on what country you are in, where (in the world) the letter is going and the value of each stamp!
im not sure if you can do this in your area but my mail carrier will let me put money in the mailbox adn she puts the correct stamps on there and gives me change
12 -9 + x = 3 -9 + x + 9 = 3 + 9 x = 3 + 9 x = 12
81
You need 9 per square foot so 9 x 12 x 12 = 1296 tiles