I don't run this sort of business but I recall reading in a book (I believe Tim Ferris' 4 hour work week) that he offered a 110% money back guarantee and in the entire history of his business only one person took him up on it.
I've not seen the 110% money back guarantee in the wild however but it feels like a memorable offer. You'll have to carefully figure out how to avoid getting taken advantage of but I feel it might be a worthwhile risk.
This is an old copywriting tactic that has been around for decades. It's the "too good to be true" policy. It's just a way to make people think "wow, how does he stay in business!". It really makes you think that their product is very good.
I vaguely remember some book author having an interesting typo related policy. His claim was, for each typo reported in his book, he would start out by paying 1 penny, and for the 2nd typo he would pay 2 pennies then 4 pennies then 8 pennies and so on. It was a crazy claim because if you double your pennies 30 times, it's over $5 million dollars. I can't remember who did this, but how's that for an incentive to buy the book haha.
Man, someone should have created a trading desk around his refund policy. I'm not sure how much his product was (or what it was), and the returns don't compound, but still.
Perhaps you'd write into your terms of business that you could only have one refund per person per year or something like that to limit this sort of edge case behaviour.
I recall it being a sports nutrition business but selling something reasonably unique. I would estimate that it would be an average sale of $50-$100, perhaps a high of $300?
So long as you're making decent sales an occasional hit of $5-$10 and the even rarer hit of $30 is hardly terrible.
I've not seen the 110% money back guarantee in the wild however but it feels like a memorable offer. You'll have to carefully figure out how to avoid getting taken advantage of but I feel it might be a worthwhile risk.