Make $4.75 USPS Flat Rate Shipping Box

This is my first instructable. I invite feedback, coaching, and correction.

My wife finally started selling stuff on the internet. One of the big things that distinguishes sellers is shipping costs. If what you are selling (in our case used boutique baby clothes) is heavier that 13oz. you have a lot of options* of how to ship. The same package could cost you $8.60, $10.95, or $4.75 depending how smart you pack it. I was able to package 2 shipments by hand in about 40 minutes. Once I made the jig I could do it in 3 minutes flat.

* Technically you have one les option, since the cut-off for the cheaper first class mail is 13oz.  If you can ship first class, you should. It's the best value. So, for packages 13ox and under, I think there is really only 1 choice.

Here is what you need.

Supplies
1. A 12 1/2in x 9 1/2in Flat Rate shipping envelope, price $0.00
2. A strip of plywood* ripped down to a width equal to the shortest dimension of your box, price $2.00 or free (scrap)
3. 8 wood screws 1 1/4in length or longer, price $1.79 or free (scrap)

Tools
Table Saw for ripping the plywood (not needed if using a 2x4)
Miter Saw for cutting the sides to length
Drill
Philips P2 bit (or T20 if using DeckMate, the only wood screws I use)
Counter sink bit (recess the screw heads, it's prettier)
Forstner bit , Wood Spade , or Auger

* You can opt for a simple 2x4 if you don't have a means of ripping the plywood, but it won't be very useful for stuffing.

Here is the paradox:
The measurements don't really matter, but they must be precise.

The width of the strip of wood should be placed against an edge of the envelope and the remainder of the envelop measured. Repeat on the perpendicular edge. Make sure to fold the flap upward so that you don't add it to the measure.

The measurements I ended up with were:
3 3/8
9 1/8
6 1/8

The thickness of the plywood was 11/16. I accidentally rounded down to 5/8. If you are going to round, round up. For your benefit, I will use 3/4 in the measurements below.
IF your wood is ripped to 3 3/8in and is 3/4in thick , Cut the pieces to:
2x  8 3/8 (that's 9 1/8 - 3/4)
2x  5 3/8 (that's 6 1/8 - 3/4)

This also assumes what I call "pin wheel" construction which I prefer because you are only screws into one end of each board. This lessens the chances of splitting. If I did the "sides inside" construction I would cut one pair to the length measured, and the other pair would be shortened 1 1/2in.

Mark and drill all of the holes before you start assembling. When you bore the hole, make sure you have a board underneath it that you can drill into so that you avoid the "tear out" that occurs from free air drilling.

I didn't bore both sides of mine. This causes a vacuum when trying to remove the jig. I will go back and bore the other side.

Don't forget what you are doing when you start putting the box together. Notice the "pin wheel" construction. Once you cut it this way, you have to follow through.

1. Use shipping tape to reinforce the corners before you start. (This is much easier to do to a flat envelope.)
2. Insert the sideways and carefully form the first corner crease.
3. Rotate the whole thing and crease the 2nd corner.
4. Remove, rotate 90°, and reinsert the jig, being careful not to rip the corners.
5. Fold in the flaps.
6. Flip the whole thing over so that the table is holding the flap closed.
7. Carefully crease the final corners.
8. Use the bore hole to retrieve the jig.

You can use the jig to estimate and shape your payload. When shipping clothing, I prefer to stuff everything into a Tyvek or zipper seal bag. It keeps the contents safer (especially the Tyvek) and dry. I then stuff the bag into the jig and apply the shipping tape in such a way that the shape is maintained by the pressure from the tape and not the vacuum of the squeezed out air.

Put your bound payload into the box and seal with the provided strip making sure to avoid wrinkles. Reinforce your seal with shipping tape. I strongly suggest using clear shipping tape (yes there is also brown shipping tape) so that your package still looks like a Flat Rate shipping envelope. Remember, if they think you have altered the envelope, they may refuse to accept it for the special flat rate. Your package can be up to 70 lbs. If it were to contain a 3 3/8 x 9 1/8 x 6 1/8 block of lead, that would be 77lbs 4oz.

If you use masking tape or office (scotch, magic, crystal, etc.) tape they will reject your package outright. Don't taunt them on this.

If you order online and print your own shipping the cost is $4.75, if you go into the post office it is $4.95.

Enjoy saving 50% or more on you shipping!