That's bull malarky don't take that kind of treatment from anyone! If you purchased through PayPal put a dispute not as described (or whatever it is) and list that they sent you the MH bulb. You don't have to mail anything back if you want this to be a bluff. I bet they will acquire and send you an HPS bulb also to solve the problem letting you keep both.
There are so many great shops available today it was really dumb of that shop to jack you around like that. I been growing a long time and I have never seen the benefit of using MH bulbs so I bet they have a surplus of them. (I use fluorescent lights in veg they build awesome vegs plants).. Of course you need HPS or equivalent for flower so of course those bulbs are a commodity.. imo