Product still under warranty,i have to pay..

i bought my son a psp and the games started hanging and the product stop reading the games,so i had the product repaired and received it back...it worked for one day then the same issue started,because of my working hours i only took it back a month later which i thought wouldn't be an issue cause they gave me a 3 month warranty on the product....they then contact us and tell us it will be R350-00 but because of the warranty they will give us a R100 discount...because there was dust and hair inside the psp which i didn't see when i took it in,the product was clean and kept in a psp pouch in my closet....so i'm not a very happy customer,cause to me it looks like a warranty doesn't mean anything here even if its the same problem again on the psp,but now a different story of why its doing this and that the warranty doesn't cover dust etc...and its only a normal service which i'm charged for....so i'm picking the product up on Friday and then ill clean it myself..if they are going to charge me just to clean it,then it's ridiculous....we have still with i the warranty period.....i don't want them to fix it if they are going to charge me for something that's still under warranty....