I took a look at the kit on the webpage that was linked in the original post. If you scroll down that page, you'll find some other products shown. One of these is a "Brown Leather Rifleman's Possibles Bag." It looks identical to the one in the kit, except it's finished (assembled), and it only costs $4.00 more than the kit.
I have not assembled one of these kits, nor have I even seen one of the kits or one of the finished bags. However, judging from the photographs, I'm not impressed. For one thing, if I were to assemble one of these, I would stitch it rather than lace it. Stitching is easier, stronger, looks better, and is more authentic. Maybe there's an antique shot pouch "out there" that was laced together, but I don't recall seeing one. So, if you get the kit and stitch it rather than lace it, you'll have a better bag. If you elect to lace it, you'll save four bucks, but will invest some time. If you don't have the required tools (including a punch), you'll need to get them.
I used to be very involved in traditional archery, and I ordered from 3Rivers Archery several times. They are great folks to deal with! I am sure this kit will make a very usable pouch and you will have the satisfaction of having assembled it yourself. However, I am just not impressed with it. I think you can do better.
In my opinion (since you asked...), you might want to at least take a look at the pouches offered by October Country. They have been serving the blackpowder community for several decades and they have a lot of pouches available in a wide range of prices.
One final thought, at risk of being labeled a "pettifogger," is that these were not called "possibles bags" until the late 20th century. Old timers called them "shot pouches," "bullet pouches," or occasionally "hunting pouches."
In any event, I hope you find what you need! If you assemble a kit or build a pouch from scratch, please show it to us!
Best regards,
Notchy Bob