Do you charge the fee when the download starts or when it is successfully completed?
If the case is the first one I suppose downloader lose his money if he can't complete because connection fails and he gets a different IP.
The payment is made first and we verify payment before delivering the content.

The link (URL) of which the user downloads from is active for at least 24 hours providing plenty of opportunity to fully download the file, and even resume failed downloads.

If the downloader's IP changes, then the link would indeed be invalid and the user would not be able to resume the download. But even if the link dropped, the user's IP should not really change, which means they WOULD be able to resume.