I have recently used your company to buy my Grandd...

I have recently used your company to buy my Granddaughter and iPhone and she is absolutely over the moon with it, through my own naivety I did not order a charger plug I thought it would come with it i then when out and bought one had you have emailed me explaining I would have ordered one from you. I also wondered If you can help me because of the quality of your phones my grandson has an iPhone 5S with a broken back case and a crack over the corner of the screen can he do part exchange for an iPhone 6 if so what would the costs be?