Naviextras is a separate company from Adventure Pilot. They provide map data to a large number of GPS device vendors. You will need to work with Naviextras to purchase a map update, not Adventure Pilot.
I have not purchased a map from Naviextras, but their site seems pretty easy to navigate. Without creating an account, I was able to click on the "Maps & More" tab at the top of their webpage, select "Adventure Pilot" and "iFly 720" as my device, and then browse their map offerings. A one-time update of the US costs $40.Presumably, you would have to create an account on their site to actually complete a purchase.
Not exactly sure how the update mechanism works. It talks about installing the "Naviextras Toolbox" software on your computer, then somehow connecting your GPS device or perhaps its SD card to the computer, and then I assume the Toolbox will somehow detect your device and handle the data transfer. I have never tried it, though.