The Black Market preffered account tech doesn't work. It shows the numbers where you buy resources as being lower, but it still charges you the full amount. I found this by messing with a mod where added more levels to all the techs, but I checked it with a pristine data folder and it was still happening. Check for yourselves, get the tech, get the cost of resources high, save up some credits and then hold down shift to buy 500 at a time, buy them when you have just a bit more credits than you need for all 500, and it will still take you down to 0 credits.

15% isn't all that much so it's hard to spot, but it's definately overcharging.