To avoid cellular data overage charges, please go to your iPad or iPhone SETTINGS app and then go to CELLULAR > USE CELLULAR DATA FOR and turn the switch OFF next to FOREFLIGHT. This will prevent ForeFlight from using cellular data for downloads or any other item that require an internet connection.
Image 1. If the switch next to ForeFlight is ON (as pictured), turn it OFF to prevent the app from using cellular data.
If you have not performed the above action and select the blue download button under MORE (MENU on iPhone) > DOWNLOADS, then a warning message will appear warning you that you are about to download over cellular data. This warning does not appear if you select a single download or use cellular data for weather related items.