Samsung - complain regarding samsung mobile

I had purchase a M** mobile dated **.**.**** by amazon , approx after month it has create some problem the resolve by servise center . after some time it has create again same problem then after lot of conversation with costomer care it has been resolved . again we have some problem with the mobile recently , its back cover opened and sevice center has charge Rs . *** for fixing it .the screen of mobile has also broken and service demanding Rs **** for repair . my complain is that whenever it is an under warranty then why demanding the charge . so it is requested please do needfull .