Poor Service and Customer Policy exchange criteria

Went to Duratec Batteries (Alberton), was told the battery was still under warranty, they did a full charge and told me the battery was fine. After 2 weeks battery was flat again, went back to them to request an exchange of warranty claim and was told it was purchased in Boksburg and needed an invoice to make an exchange in Alberton. The car was purchased second hand without any proof of invoice of course. They did the alternator test, and tested 100% fine. The battery is flat again, how must I travel back in forth to Boksburg after it could have been replaced the first time Duractec (Alberton). Please Hellopeter may you with this situation.