I have not been able to find anyway to buy multiples of the same thing to count towards the discount, either. I wanted to buy a couple of character slots and a couple of shared storage upgrades, but since they can't all count towards the same discount I decided to wait and see if they go on sale. I think it's disappointing, though.
Celordal: 105 RK, Tiluriel: 105 LM, Jaenoviel: leveling captain, Galynna: leveling champion, and a few other toons -- Runic Knights, Brandywine
Other alts -- Riders of the Great Eagles, Brandywine, originally Riddermark