False advertising

i bought a phone at cash crusaders have not used it for even five months the charging block gives i take the phone back few weeks after i follow up the person Nene with email address: *** tells me that they repair Centre say it is uneconomical to repair the phone so what i wanted to know: My cellphone is with you for 18 out of 21 days for repairs please forward me the said invoice which determines that I am to blame or that I have invalidated my warranty somehow.So that i can budget accordingly if i find this invoice sufficient.