I had a SNAFU similar to this on my site as well, in which someone paid for a box of my peppers, but because of a glitch with the Paypal IPN (Instant Payment Notification), my product stock wasn't reduced, and someone ordered the same thing a few days later when I had no pods to ship. Thankfully, the person was cool with waiting until I had pods ready and Paypal since fixed the IPN issue, and it's working fine on my site now, and looking at my order history, there were also some cancelled orders because the IPN didn't reply in time, so my site auto-cancelled the order for a lack of an IPN response (no money was ever sent to my Paypal account)