If you go in-store, they should give you the $50 refund since you're within 15 days. If they refuse, make their job more difficult. Tell them then you want to return your phone and cancel your service (since you have 15 days to do so) and sign up again and get the 5C for $50 cheaper. Plus new phone all together. Wanting to do the latter should get them to do the former anyways.