Through the App or your MyUsage Web Page you can view under Billing, previous Bill History and Transaction History
Sample bill history (click on the down arrow to get more details).
You can see my payments I make to ensure that I am not paying a bill.
I am not aware of anyway that you can view any emails or letters that have been sent.
The onus is normally on the receiver to keep or retain any important documentation.
If you would like to speak (communicate!) with AGL then you can CONTACT AGL by clicking on the link here and decide which method is best for you.