Hi Thomas!
It depends on your server configuration and what software you are using.
An underscore isn't negative. At one point Matt Cutts recommended using dashes (-) as a delimiter, but that was many years ago and he has since said that Google is smart enough to figure out what delimiters you are using.
I find that there are diehard fans of specific delimiters, which I think is silly. I myself prefer the dash, but again that's just personal preference.
The more concerning matter is that you have no sense of hierarchy in your URL. I'm assuming you are running an old version of osCommerce or something similar (your website won't load for me). I don't have much experience in osCommerce but I do remember that their permalink structure was horribly limiting back in the day. Not sure how plausible it would be to change your structure, but a quick Google search for your version number should return an answer rather quickly.
Also, both of your links are the same. I think you meant for the second one to be different 
Would be glad to help out further if you can supply more information. Cheers!
P.S. - I'm biased toward Magento, but they have a free Community Edition you can check out if you want an eCommerce solution that isn't antiquated.