My acer Iconia tab 7

My Acer Iconia Tab 7 was bought at incredible connection 25th February and has since been in at Acer twice where the main board was replaced. Initially all documents went with the device to Acer and never returned. The second time the device went in I was not asked for proof of payment. It has now gone in a third time and they need to replace it as it cant be repaired and they are insisting on the proof if payment which they never returned to me. Stuart says he cant help me and i need to get a duplicate proof of payment. Which they have misplaced I use this device daily. Its costs me in excess of 250 rand for WiFi each time to set the device up again. Acer does not care.