I just set up a store selling cellphone cases and some accessories including screen protector, and I'm running a promotion and would like to offer free screen protector with every case purchase, is this possible to set up ? (ie. A screen protector gets added to the shopping cart at $0 automatically when a case is put in the cart, and if the case is removed from cart, the screen protector would be removed from the shopping cart automatically)

Thank you very much in advance!!!!!