Is there any replacement charge levied on the century mattress within the guarantee period ?

On 2nd October 2010 i have purchased a century mattress at Raja Furnitures, RTC X Roads, Hyderabad, Andhra Pradesh. After using many days i have found some defects on the mattress. They gave me 6 months guarantee period. So i contact the dealer and submitted my mattress for replacement within the guarantee period on 21st February 2011. They said that it will take one week time to replace. After 5 days they called me and replaced with new one. But they have taken a replacement charge of Rupees 500/-. I asked them that why I should pay 500 rupees within the guarantee period for replacement. They told me the company will charge us for replacement, so we are taking it from you. Is it correct to collect replace charge from the customers within the guarantee period? Please help me.