In fact, the website even continues to state that there are no additional charges as long as I subscribe to Guardian Angel Support.

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Interface charges are for Amazing Charts' connection to the third-party product only. Many of these third-party products cost money themselves, and/or may charge more for their side of the interface to an EHR. Interface charges do not recur if you subscribe to Guardian Angel Support & Maintenance service (so you can get the updates and interface enhancements as they are released). Non-subscribers to our maintenance service will be charged annually for these interfaces. - See more at: http://amazingcharts.com/products/transparent-pricing/#sthash.ZE1U6Ed9.dpuf