We looked at the ones made for wheelchairs when DD got her power chair. But, because they are made for wheelchairs, the ones we saw were about 3 times the price. I'll be interested in the brand you found, Linda. I'd like to see how it would be planned out compared to a backpack.
We have always just used a regular backpack. You can find everything from a very simple one with only one big and one little pocket to backpacks with a separate pocket for almost anything you might want.
There's also a style of backpack/purse that's popular right now that would work well on back of a wheelchair. It's sort of just a pouch with drawstring straps to keep it closed. I can't explain it better, but if you go to the purse/accessories department at Target, they have some in bright prints for less than $20.
If you know anyone handy with sewing, you could also make a really plain bag like the volunteers at the hospital where I work make for wheelchairs and walkers. They use upholstery fabric and it's basically just 2 rectangles of fabric sewn together along 3 sides. You hem the top and add a strap on each of the back corners to sling it over the wheelchair push handles. If you want to be fancy, you can add a zipper or some velcro to the top to close it.