Refunds are just a fact of life when you do business. Guarantees generally increase sales significantly and only a small percentage of people will request a refund.

For example. Lets assume you sell a product at $47 and usually sell 100 per month. Let's assume adding a guarantee will double your sales, but 10% of people refund (a very high rate - much higher than most real life cases).

Your income without a guarantee = 100 @ $47 = $4,700

Your income with a guarantee = 200 @ $47 = $9,400 minus refunds of 20 @ $47 = -$940
$9,400 - $940 = $8,460.

So you made an extra $3,760 in income even though you had a 10% refund rate.