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how many stamps for a 3.4oz envelope
what stamp amount is needed if the letter is 15 oz.?
That depends on where you are sending it from (you country of origin) and to (the country of destination).
Currently in the US up to one ounce is 45 cents. So however many stamps it takes to make that amount.
how many stamps for a 10.4 ounce envelope
As of 2016, a 2.5 ounce envelope it would cost 93 cents (one Forever stamp plus two extra ounces).
Two stamps
For a 3.4oz envelope you will need to purchase 94 cents worth of stamps. If you go to a post office they will charge you 94 cents, place the stamps on it for you, and there is a drop box right there!
After 3.5 ounces, the item moves up to large envelope rates. The current rate as of 2016 for an 11 ounce large envelope is $3.18.
At that size and weight, it would cost $2.20 (as of 2016).
It depends on the weight as it would be processed as "Flat Mail". Visit the Related Link for a chart of the price by oz. from the USPS.
In the UK, it is not only the weight, but also the size and thickness of the envelope or package, and the destination (UK or abroad). It is best take take the envelope to a post office to ensure the correct postage is used.