A 6” x 9” envelope weighing up to 1 ounce requires one $.50 first class rate stamp. For each additional ounce, you’ll have to pay $0.21.
So, for weight between 1 and 2 ounces, it’ll cost you $0.71. Weight between 2 and 3 ounces will cost $.92 and weight between 3.0 and 3.5 ounces will cost $1.13.Envelopes that exceed 3.5 ounces in weight or are thicker than ¼ inch will be charged the larger envelope rates.If the contents of the envelope have an important business or correspondence value to either the sender or receiver, there are additional postal services available for added fees based on the envelope size, shape, and weight.A 6” x 9” envelope will fit in a USPS Priority Mail 9-1/2” x 12-1/2” envelope and can be mailed for a domestic flat.
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