Item not received despite proof, but PayPal refunded buyer anyway :(

Per topic… I sent a buyer a digital unlock code for my software by email several times (which never bounced) because they kept claiming they never received it. My site's "Contact" page even links to my company's subreddit so the buyer can discuss the issue there if email isn't working for them, but PayPal decided this wasn't enough proof and refunded the buyer anyway.

I looked at PayPal's Resolution Center for an "Appeal" link but there isn't any. At the end of the day I guess I don't need such a liar as a customer anyway, so I let it go. I only lost $40 so such is life. But still, this bugs me that PayPal doesn't care what proof the seller provides and automatically sides with the buyer. 😦

Any ideas or tips on how I can avoid such claims of non-receipt in future? Thanks.