Legit? Sure, they can charge you for anything they want.
But it's a crock.
I'd guess that they've never advertised the Prius in the local paper ... and you might want to point that out to them.
If they're unwilling to remove the charge outright, ask for something in return ... tell them if they're willing to remove it you'll buy, oh the rear bumper applique and the cargo net and floormat set. That way you get something for it and they can glean a little profit.
The trouble is that at the moment it's a sellers market and they're really taking advantage of people. So if a $300 upcharge is all you're in for, some might consider you fortunate.