Step 6
Now we just created shadow effect .But there is one problem. The problem is, that blurred shape. To see that problem change the background color black to white. And now you can see the blur shape is blurred outside from top area.
To remove it, click on rectangular marquee tool > select that blur layer > select it > and press Del.

Step 7
Now our envelop looks nice .But need to give more effect. we want to show a flap is opened slightly . Select that triangle shape layer .Right click on it > click on blending options > check gradient overlay > change gradient color > click on black color stop > change it black to white #ffffff.and change white color stop ,white to gray #dddddd.then press OK to close both windows .Change background color white to black again.

Final Result:

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