We sell software online using PayPal. Occasionally PayPal will hold up the sale for mysterious reasons for up to 5 days. Then we have to tell the user that sorry, we know we just need to send them an email with a license key but PayPal are holding it up. I guess payment systems are just slow, probably for security/fraud prevention reasons.
We do sell software, too. But we send the key immediately. In case the payment gets declined later by our processor we just lose one license which is no real cost to us. But a good customer experience is important enough to us to warrant this risk.
