Here are some test results from a quick mock-up. All are at f/5.6 and 1/250.
Top left - 400 ISO, bare flash, Top right - with folded white A4.
Bottom Left 100 ISO, bare flash, Bottom right - with folded white A4.
I guess with the right environment and settings you might get some worthwhile results but I think the DIY option would be the best bet. Note the shadow line in the top right photo. You would need to think carefully about the design to ensure the light went exactly where you needed it, and not where you didn't.
EDIT : Actually, I've just looked at the enlarged photos on the website -
Large Image Adorama Sku#FALSUW - and it looks like they are using mirrors to bounce the light upwards, not just simple paper/card/plastic, so I guess the efficiency would be somewhat better than my test results - quite a bit better perhaps.