....Are you Serious?!?!?
As far as I know the only thing that has been associated with HID lights and plant toxicity is PVC, and Only if its processed in a certain way, like coating the inside of the cheap tents.
I really dont think you will have any issues with the shelf you bought.
But if your worried and you want to be 100% sure I would put the shelf, or a piece of the material about 5-6" from your HID light and leave it there for as long as you can wait. 10 hours, 12 hours, 18 hours, check on it, see if its getting hot or deformed, or even just weak and easier to bend. And obviously if you smell it burning under the HID that would be a problem.
I think it will be fine though. How big is your light anyway?