Generate receipt/invoice of payment

This should be given high priority for financial reasons. Some of us can get Audemic paid for by our employers, but this absolutely requires a receipt or invoice. With this feature, the Audemic user base could grow significantly if researchers can get their institutions to pay for the service, which would result in an influx of funds for Audemic and improved features for users. (Please note that both are important for financial service departments: an invoice is before payment; a receipt is after payment with proof of payment.)