Step 1: Collect the Materials
1. some paracord, twine, string, shoelaces, bootlaces (not strawberry), ripped bits of fabric, anything that'll hold what you're carrying (shouldn't be too heavy anyway - you're putting it in a plastic grocery bag!) Mine was approximately 7'5" just because that's what i cut it to. I have lengths of paracord like this stashed everywhere. You'll need about that length anyway. So tie stuff together, whatever you have to do. I won't judge.
2. two coins, pebbles, candies, marbles, quail eggs (maybe not), anything small, not sharp, and having a shape. I used pennies because i wanted to use the cheap pun in the title.
3. a bag. great way to recycle! also most businesses give you this if you purchase something anyway...if you forget to tell them not to bother. don't worry, this time it works out! two handles on the top are really handy, although you _could_ do this without them...but i won't be going over that.
4. something to carry. sharp edges are bad, as is excess weight. you can double bag if you want.
Step 2: Drop the Money
Step 3: The First Knots
Place each "noose" around the scrawny neck below the penny you've placed in the bag and tighten. Do this on both sides. You should now have a bag with the same cord tied to each corner. Almost there!
Step 4: Placing Your Goods, and the FINAL KNOT
Next, take the cord that you've already attached, grab it in the exact middle, and thread that middle through both handles. pick an appropriate length (the bottom sections of cord will form your shoulder straps) and tie a regular slipknot with the doubled section (the part that you threaded through the handles). It's just the opposite of the "noose" that you tied for the bottom ones. This way you pull on the doubled section to untie, as opposed it untying itself.
EDIT: For ease, I've added some more pictures. It's just a regular doubled slipknot, but the original pictures looked like a NYC subway map of paracord. So, for reference, in the upper right corner of the photos, i've simulated the knot in just one strand.