Lost iPhone receipt

I did an express exchange with Apple recently from my iPhone I returned the defective iPhone as soon as I got the new one and restored it. I use the label. Apple sat with the phone charger from FedEx to return it. Turns out Apple did not get the correct phone. I put the phone I purchase in there but that’s not what they got so they sent it back to me understandably now they want the original receipt from three years ago. Those receipts come by email when you go to the store to buy a phone. Which is fine except when I restore a phone on my back up data is never fully restored. There’s missing data in every app and every part of my phone. I cannot find that receipt at all. My email he’s only halfway there. I have big gaps of months at a time. Now Apple wants to charge me for the full price of the phone that I recharge. I had no option to insure it. I got no receipt when I handed it to the FedEx man. This seems very unfair. I did exactly as Apple told me and still I’m going to be out $1400. You think Apple would have a way to track and identify phones better than this. What can I do?