Actually, it's quite do-able for many sites. People go through the whole checkout process and the order is processed in ZenCart normally. Only the payment is handled by fax, not much different operationally than accepting a money order. You can set up a default order status for fax orders (such as "pending fax payment") to avoid mistakes (like shipping without receiving payment).

If I interpreted his post correctly, Magicman wanted ZenCart to store the order and prepare a prefilled order form plus cc authorization all on one fax document. If something like that hasn't been created yet, it would seem relatively easy to create by hijacking some code from invoice.php to create the prefilled order form and adding the credit card text.