I just checked it — Bitcoin Fax is a real service for sending faxes paid via Lightning. The concept is legit (pay-per-fax with no signup).
That said, I'd recommend a small test first: send a cheap fax to a number you control (or a free fax-to-email service) to verify delivery before trusting it with anything important. The Lightning payment is instant and non-reversible, so confirming it works on a low-stakes test is worth the few sats.
