In addition to my standard picture - I have actually taken a little time to create a picture tutorial, as promised.
Step One:
You can use any envelope. I used a #10 business envelope because I can get two bags from one envelope. If you use an invitation envelope, you will only get one.
First, seal the envelope, then cut it in half.

Step Two:
Decorate your envelope.
First I ran it through my Cuttlebug using the Polka Dots folder (had to, I just bought it a few days ago)
Then I stamped with Grey ink and flourish stamps. You could do this before the Cuttlebug, but I wanted a rougher look from the dots.
Step Three:
Score the three edges at .5 inches(you can do larger, just make sure you do all three edges). If you are using an invitation envelope, you will score three sides and cut off one of the ends at the same measurements.
Fold at the score lines and burnish with a bone folder or smooth ended tool. This is a little tricky due to the embossing, so pay attention to the fold lines.
Step Four:
Open the almost bag and insert your hand to help poof it and adjust the fold areas as needed.
The two corners will poke outwards. Flatten them and glue them to the bottom of the bag.
Step Six:
Fill the bag with treats and attach the Topper to the bag. I glue the Topper to the front and back to keep the bag closed and to prevent "snitching.