It wasn't completely solid. It had a CV joint on the front that had a small amount of movement. But I agree, they should have known better because it was not cheap.They are available. Every 2005 - 2014 Mustang has fixed flanges on the trans and diff and a sliding driveshaft. The one I linked to has a sliding spline under the black bellows. Mine is a Spidershaft (out of business) and has about 2 inches of travel, similar to the std slip yoke on older transmissions and is aluminum. Driveshaftshop.com can make what you needed. But I guess it doesn't matter now. I just mentioned it so that you knew they existed in case it comes up again.
I'm surprised the driveshaft company would sell you a solid driveshaft. They should know better.