About six dollars
The cost to mail a manila envelope will depend on the size of the envelope and the weight of the envelope. The average cost to mail a manila envelope that is 8"X11" is about $2 if the contents weigh less than one pound.
Where to, and from which country. Costs WILL differ.
In a standard size envelope, as of June 2009, you need 75 cents in stamps to mail an envelope up to 1 ounce to Canada. You could use two 44 cent stamps (you would be paying too much) or you can buy 75 cents of stamps at a post office. You need an additional 25 cents worth of stamps for each additional ounce or fraction of an ounce. Letters over 3.5 ounces have different rates.
how much a first letter that weighs 2.1 oz cost to mail
Assuming you are referring to First-Class Mail, the cost of shipping a 2-ounce mail piece via the USPS will vary depending on the size of the mail piece. Assuming that it is a letter, it will cost $.61. If it is a large envelope or a flat mail piece, it will cost $1.05. Chuck Stamps.com
The first ounce is 44 cents. The second and any additional ounces would be 17 cents. This comes to 61 cents for a 2 ounce letter.
A 2 ounce letter costs $1.10 to mail from New Jersey to Ontario, Canada. A 4 ounce letter costs $2.65.
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From May 2009 to late 2011 in the US, the one-ounce first class rate was 44 cents. The rate for a second ounce was only 17 cents, for a total of 61 cents. (Assuming that the letter is neither a large envelope or a thick envelope, which are charged at different base rates.) Major mailers were eligible for reduced rates for pieces between 1 and 2 ounces when they included a return reply envelope.
Yes, you can mail a 2 ounce letter with that much postage and be paying more than you should.
It should be 61 cents: 44 cents for the first ounce and 17 cents for the second ounce.