As we have explained many times, the bookkeeping and the accounting of all businesses in Germany must be well documented. All financial transactions must not only be recorded but have each to be provided with a document (an INVOICE, RECEIPT) which reveals why the financial transaction belongs to the business and not to some private pleasure. The German tax office tends to see undocumented financial transactions as private, not relevant to the business, with all unpleasant consequences for the business with respect to tax returns, etc.
As many of our clients have great difficulties in distinguishing between relevant and irrelevant documents for taxes and accounting, here a short advice:
On the (digital) paper you might be considering a relevant document for a certain financial transaction, SEARCH FOR AN INVOICE NUMBER (Rechnungsnummer)! If you can not find an invoice number (unless your paper is a receipt for a really small amount), this paper is NOT an INVOICE, but probably an order confirmation, a payment confirmation, a reminder, etc. And the document must have been issued by a VENDOR OF GOODS OR PROVIDER OF SERVICES, not by some financial institution - bank, Paypal, etc.
THUMB RULE: NO INVOICE NUMBER = NOT AN INVOICE = NOT APPLICABLE FOR TAXES AND ACCOUNTING
MBA for Finance and Financial Services (UK), Steuerfachwirt (GER)