You can download your monthly reports one at a time from the Billing History tab and play with them in Excel or just print them out as proof for tax purposes.
As to billing, it all depends on how you do your online purchasing. AFAIK, you don't have to pay every Amazon charge with the same card, and you should be able to assign a different card at any time. If you have multiple credit cards, as many of us do, it might be convenient to use one card only for writing business costs and use a different one for personal purchases.
The classic wisdom has been to get a separate business bank account, but for people at the prawny stage of self-publishing that's a completely unnecessary expense. You could probably do it all through one free online checking account.