Bad quality beds

Good Day I bought two beds in Phoenix branch last year in November 24. The mattress started to sink and deepen and I went back to the store to report in Febuary. I was told to bring the invoice as proof of purchase, unfortunately in nowadays we do not keep pieces of paper we do not even have wallets/ purses we use our phones most of the time. I lost the invoice. I went back to the shop again in May I told them that the proof I have is the bank statement with their banking details when I was do the transfer. I was told to contact the Customer Care office I did that in July. I was told that they can't help me because I do not have the invoice even though I did send them my bank statement show the payment history. I do not know what to do anymore now. Please assist