We use Paypal. We offer a monthly subscription service. We need to send out (1 hour time delay) automatic branded receipts once we receive payments. The receipt should included amounts received, confirm shipping address, and include our logo. We would also like to be able to change the email template from time to time. Also, the customer should be able to unsubscribe. This data will also need to be password protected on our server.
Customers subscribe through our Paypal buttons, these people would be who we want the receipts sent to. We receive other payments direct by email - these do not need the branded receipts.
Our project knowledge: this will require PHP and IPN work (and other).