Very clean site. osCommerce has a full admin section and you should have complete access to it. If your designer has not given you that then you should move the site and change designers.
If you are talking about programming for it, then I would go directly to the source.
http://www.oscommerce.com/
This is an open source application. If you can not find modules or patches to do what you want, you can most likely find someone from within the site to do it for you. Some people will do it for free and just offer it up for the credit of creating it.
Best of luck.