Whatever you buy has to have a Mopar part number. But not necessarily for a Challenger. I got some floor mats and remote start for my 2006 Charger. You can also buy maintenance parts, such as oil filters, etc. If it's in the Mopar parts book, you can get it.
I believe you have 30 days after the purchase date to get the accessories.
Not much to choose from. I ordered some replacement parts for down the road. New floormats, exhaust tips, fuel filler, and splash guards. I already had this stuff, now I have extra for when my kids restore it in 30 years. Really wasn't anything I wanted, and this beat wasting the bucks.