I really like Verifly. I use it on a per-flight basis. They are affiliated with legit big aeronautical insurance companies, and they keep my clients happy with the instant documentation for each each flight. It's easy to use my phone to add additional insured, up the coverage, increase the flight parameters, and make other adjustments. It's great that you can now book ahead, and cancel up to an hour before the scheduled flight if the weather turns bad. Perhaps not the cheapest, but I like tailoring the coverage for the flight and the client, and I wrap the cost into my bill.