That has nothing to do with Firefox - this requires you contact your CC issuer and downloadhelper support to resolve this problem. This is not something for Firefox forum End User help to resolve.

The Video DownloadHelper extension is free to install and use. https://addons.mozilla.org/firefox/addon/video-downloadhelper/

I found a charge of $28.50 on my credit card from Saint Gervais Sur Mar with something to do with DownloadHelper. I do not remember that charge and believed it was free. How can I cancel any subscription and get money back?

Perhaps you inputted your credit card information to get a premium license for unrestricted usage. https://www.downloadhelper.net/convert