It does not cost very much in order to ship a standard piece of US mail in a padded envelope. The cost of shipping standard US mail in padded envelopes can cost as little as $3.
Calculate it on usps.com
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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.
Anything that can be sent in the mail is suitable for padded envelopes. Sharp objects could puncture a padded envelope, so those should be avoided when possible.
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The cost to mail a 5.75 x 8.75 envelope will depend on the weight of the envelope. As of May 2014, the cost to mail a standard letter is 49 cents.
Those dimensions make it a large envelope, so postage starts at 94 cents (two Forever stamps) for the first ounce, and more if it's heavier.
Padded envelopes may be sent in the mail for many reasons. The main reason is to ensure that the contents of the item in the envelope do not suffer from breakage.
Postage depends on which country you live in, how much the envelope and contents weigh, and where you are sending the envelope to. You'll need to include all that in your question to get the correct answer - or you could just go to your local postal website and get the answer from the experts...
It currently costs 44 cent to mail a regular envelope, which is the price for a stamp.