As a lot of others have mentioned , Ive done this several times without problem and out of close to a hundred I've had only one not make it , or that's what the buyer said anyways ...
By doing just as other mentioned and simply tape the 2x2 inbetween a piece of cardboard that I cut and make into a birthday card type shape ....
I usually fold a small piece of paper around the cardboard 2x2 before taping it to my cardboard that way when the tape is removed it doesn't rip or leave residue on the actual 2x2 holding the coin ...
One thing I will suggest , is not to do this with a coin that is worth more than 10 bucks ...
I only do this with lower value coins , as it's tough to sell a coin that's worth less than 10 bucks if you have to charge almost 3 in shipping ...
I've never sent anything larger than a quarter this way , I have actually sent 2 quarters , or 3 dimes in the same envelope several times that have made it without a problem ...
I do however use a 71 cent 2 ounce stamp rather than a 49 cent stamp even though most of these envelopes weigh less than an ounce this was a suggestion from my local postal worker ...