I've been making elaborate Halloween costumes for my kids for 15 years. This was the simplest, quickest and least expensive costume I've even made and even made two -- one for my daughter and one for her friend.
For one costume, you'll need three white poster boards (I used extra thick and they cost about $1.29 each), Black Duck Tape and Rope or Ribbon for the handle. I also bought stripped tissue paper on Amazon for $9.00 and black ribbon to create suspenders to hold the bag on.
Step 1: Assembly
I used two of the poster boards to create the front and back panels. With the third board, I cut it in half and then folded each half in half. This created a crease to look like an actual paper bag.
I then used clear packing tape and taped each piece together, creating a long string of four board. Using the black duck tape, I created stripes. For the first bag I did, I eyeballed the whole thing, but for the second, I made light pencil marks to act as a guide. The second came out better.
Once the stripes were down, I tapes the two ends tighter to create a cube, or bag. I used a hole punch to makes holes for the rope handle and then taped tissue paper along the top rim. I also taped in black 2 inch wide ribbon to act as suspenders. They were crossed in the back to stay on better.
For the word "Sephora" I simply printed it out on the computer using Ariel Narrow font in 137 pt. I did add a 12 pt space between each letter to spread it out just slightly.
Making two of these took less than an hour.
Step 2: Trick or Treat
The girls wore all black under their shopping bags and off they go to Trick or Treat!