What is the safest way to transmit credit card info

Neither is inherently safe. Fax is too open (anyone can see the incoming document), and most people don't have easy access to a fax machine. Email is easily shared/forwarded. I mean, you could elect either method, but that places a lot of trust in the contractor, their office, and their process. But email is easiest.

Generally? For something specific? To a merchant?