Do NOT rely on auto return for your DB processing, always use IPN for this as there are to many situations where the auto return is never completed but an IPN post will always be sent from PayPal whether it is a second or an hour after the transaction.
What I would do in your situation is on the page that you return the customer to after the PayPal transaction is completed is explain that they will get an email confirmation/receipt as soon as the payment is cleared.