Will you be using wool or cotton for the braids? If you are using wool, using the method Nola suggested would make a lovely matching handle. Very polished and sophisticated.
If you are using cotton, I was thinking that you might want something inside the fabric to make the handle less flimsy. Have you ever made corded spaghetti straps for a sun dress or evening gown, or even a nightie? That is the technique I would use.
Nola is spot on about experimenting with the length. Three times is a good starting place. If you need a more exact measurement, start with a sample piece either 10" or 20" long. Make it up (stitch and braid), measure how much it shrunk. From that you can calculate the percentage that is lost in length by braiding. Enlarge the raw length by that percentage and you should snd up with the desired finished length. I mention this only as you are selling the purses, and I am sure you want to keep waste to a minimum to keep your overhead down.
To make a corded strap, start, as Nola says, with the strip of fabric, cut to length plus seam allowences plus a couple of extra inches that will be cut off later. The width will be determined by the size of cording you select. (Cotton cording can be purchased in the upholstery section of the your fabric store.) Fold the strip in half length-wise, right sides together.
The way I was taught to do this was back in the day when silk covered macrame cording was cheap and plentiful. Nowadays it can be hard to find so I have devised my own method. I will share both with you and you decide what you are most comfortable with.
The old way: Cut lengths of cotton cording and silk covered cording two or three inches longer than the fabric strip. Place the silk covered cord inside the fabric along the folded edge. Place the cotton cord so an 1"-1 1/2" is also inside one end of the strip. The rest of the cotton cord will be streaming out beyond the fabric strip. Sew through the fabric and both cords several times to anchor them. Then carefully sew up the length of the strip, encasing the silk covered cord.
Now think in terms of zoology. That little bundle where the cords are stitched inside is the rattle on the snake. You are going to pull on the silk cord and carefully pull the rattle out. As you do this, you are going to be working the fabric "skin" back up over the "skeleton". When you are done, cut off the rattle and discard the silk cord. The seams are completely encased in the cording.
My way: I do exactly the same thing, except I use no silk cording. I pin a sturdy safety pin through the cotton cording so that the pin is inside the folded fabric. Secure the pin far enough into the cord so that the cord will not unravel when you pull it out. Make sure the pin is smaller than the width of the cord so it will feed through easily after the strip is stitched.
When the stitching is done, I grab the whole "rattle" and work it up to the mouth opening, working the skin up over the skeleton as I go. Cut off the rattle and I'm in business.
You might want to do some experimenting. I think using bias cut strips will give you a nicer looking finished product. It will lay smoother over the cording and will be less stiff and rigid for the braiding.