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 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).
how many stamps for a 10.4 ounce envelope
The number of stamps required for a 10 oz envelope (7x11 inches) depends on the postage rates set by the postal service. As of my last update, for domestic mail in the U.S., a 10 oz envelope typically requires a minimum of $5.00 in postage. You can use one or more stamps to reach this amount, depending on the denominations of the stamps you have. It's best to check with your local postal service for the most accurate and current rates.
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.
For a 2.2 oz envelope, you would typically need two Forever stamps. The first stamp covers the standard letter rate for up to 1 ounce, and the second stamp covers the additional weight up to 2 ounces. Since the 2.2 oz envelope exceeds the 2-ounce limit, an additional ounce rate applies, which requires a total of two stamps. Always check the current USPS rates to confirm, as they can change.
At that size and weight, it would cost $2.20 (as of 2016).
One forever stamps pays for up to a one ounce letter. BTW, I think .06 oz. is impossibly light. Maybe you meant .6 or something else.