I’d go with something like this…
"Hi Mr or Mrs Customer. Many thanks for buying my item! I’ve received your refund request with the reason for the refund stating that you are having a problem with item support. I notice from my records that no support requests have been logged from your account. In most cases, refunds won’t be issued unless the item is faulty or unfit for purpose, so it’s not our policy to issue refunds prior to following basic troubleshooting to see if that’s the case.
However, I’d be more than happy to assist with any problems you are having… all you need to do is open a support request at the following link, and then we can go through the necessary troubleshooting steps. I’m confident we can get you up and running in no time. I look forward to hearing from you."
Then if they argue you could whip out some kind of smartphone analogy… I.e. "I can appreciate your frustration, however, it’s important that we follow a specific procedure when it comes to troubleshooting. We can’t just issue refunds upon request. Like if your smartphone isn’t working properly… you’d send it to the manufacturer so they can troubleshoot, fix, replace or refund. They wouldn’t issue a refund or a replacement without seeing the phone first, and finding out what’s wrong with it. Anyway, I’m standing by and ready to help. I’ll keep an eye open for your support request, and once that’s logged, hopefully we can get this fixed asap.
If you’re not willing to follow troubleshooting steps then I apologize, but we won’t be able to issue a refund. If you’d like to dispute this decision, then you should contact Envato support."
I think the key to declining a refund is to be polite, state the facts, keep it friendly but professional and don’t get emotional. Keep in mind that if it does to go to support as a dispute, then they’re probably going to read your conversation, so if you’re screaming and swearing at the buyer then it;s not going to reflect well on you and support might issue a refund just to keep the buyer quiet. I’m not specifically telling you this, I’m sure you know what you’re doing, just for others that might be reading. You never know… it might come in handy to somebody!