I have a saying for that:
One convinced against their will. Is of the same opinion still.
IOW don't sweat it. Some people live to complain. They're bitter people just looking for a place to vent, and you just happened to be that vent this time. Don't worry yourself over it. As you said; you work hard to get it right. You know you did the right thing, and there's probably nothing you can say or do that will change their mind (like I said above).
I find the best policy in these situations is to always be polite, and try very hard to sound empathetic to their concerns -- even if you're not.
eg;
I'm really sorry to hear you feel that way. That's probably really good advice. But do know I try very hard to be as accurate as possible. Thanks for your feedback, and thank you very much for your purchase.
At this point you've pretty much ended any further argument. You've taken all the wind out of their sails.
Now they think they've won the battle. But you know better. 😉
Best wishes.
