Ran a business selling one-time digital purchases for years and have talked with many, many other people with similar businesses. We're basically unanimous on this: offer the guarantee, reap the marketing win, pull less hair out when dealing with very annoying customers. They have an attractive backstopping option if you don't offer the guarantee, which is calling up their bank and saying "Internet merchant didn't deliver as promised, give me the money back", which will get the refund anyhow and ding you on a fee (for processing the chargeback) and increment a counter at the payment processor that you very keenly do not want incremented above a certain number.
(Above comment in my personal capacity, not speaking as someone who is very professionally involved with that last clause.)
(Above comment in my personal capacity, not speaking as someone who is very professionally involved with that last clause.)