If you can't remove the glass (which is a great idea btw) and if you're using a DSLR camera, invest in a polarizing filter. It screws on to the end of your camera lens, and you turn the filter until you can't see the glare anymore. They work just like polarizing sunglasses, reducing glare and reflections. They're not super cheap, but if you're investing in your shop, and if you can't get rid of the glass in the frames (which would be my first choice) then a polarizing filter will be worth the investment.
This article has more on polarizing filters, it's mostly about landscape photography, but it will give you an idea of how they work.
http://www.luminous-landscape.com/tutorials/polarizers.shtmlhope that helps!!