Just to clarify; Only the download is free.
This app charges a fee for page ranging from $.25 to $.50, per page depending on how many "credits" you purchase in advance (more is cheaper) multiplied by cost factor , depending on the country you fax to. For US domestic the fact is '1' meaning the cost is between $.25 to $.50 per page. In order to get the $.25 rate you must purchase a minimum of 200 "credits". So for the occasional faxer to a domestic location, the cost is generally $.50 per page. There is a fee table on the website under "Pricing", just above the "Contact US" section of the webpage near the bottom.
While the developer states that "credits never expire", something to keep in mind when purchasing is that the 'expire promise' only stays in effect so long as the developer stays in business.
As for myself, I continue use a USB fax modem I have had for years, on a Mac running Sierra by using VMWares Fusion emulator to run Windows 10 which still supports fax modems.
Given that many financial institutions still require faxing for important documents for security reasons, I still strongly feel that it was a terrible decision for Apple to discontinue supporting fax modems in the current versions of MacOS.