If you look at what google does, they already offer you an admin panel where you can see all the information recorded about you, and you can download it, etc.

If you look at what google does, they already offer you an admin panel where you can see all the information recorded about you, and you can download it, etc. How is this much more of a hassle than being required to send people are receipt as proof of purchase... Ensure customers can see what you record about them when logged in (probably in their user profile), then minimize what you record to what you need.