My immediate thought is to have a 'universal review code' on your invoice, which somehow encodes the monetary amount of business as well as the date. Reviewers could use this code when leaving a review on a 3rd party site to prove they were a genuine customer. There's an opportunity for stalking or harassment if the business was able to correlate the code to an individual though, so the money and time might have to be bucketed. If a customer got a refund the same system could be updated to show the fact.
