one stamp is good for one ounce. packaged by weight not by volume. Size doesn't matter.
a bubble weighs one milligram
a 7x9 mailer weighs roughly 1.5 ounces
3
The cost of postage depends on how much it weighs and the country where you are mailing it and its destination.
17 cents
20 cents
How much postage is required for a 6x8 inch card? It weighs less than 1 ounce.
Domestic postage is $1.39 for an ordinary envelope of uniform thickness.
The postage is standard first class 1 ounce (a "Forever" stamp).
Go to the USPS website.
It would be 44 cents for the first ounce and 17 cents for the second ounce (or fraction of the second ounce).
Postage to Europe from the US is 98 cents for a one-ounce letter.