Originally Posted by MSPeconomist

Is this a consolidator or gray market travel agent? Normally Z is indeed upgrade inventory while C is a relatively expensive but not completely full fare business class bucket. My instinct would be to become very suspicious that the travel agent could be using illegally purchased miles. Can you see your exact fare codes or fare rules? Can you see your bookings and receipts (doubtful) on delta.dumb? How do the fares you pay compare to DL published fares for your routes, especially near the time of ticketing?

I assume that this is not your corporate travel agent, who might well have special contracted fares, but those generally seem to book into normal inventory codes. What do you know about the travel agent and how did you discover them? Have you tried to do some obvious google searches for problems, in luring BBB?