Energy cells, and all other cargo, are bought one at a time by default. Container class has to do with the kinds of ships that can carry them, some smaller ships can't carry larger cargo, like oar. Your mercury is rated for cargo up to XL, extra large class, so can cary those reliably.
On another note, I just discovered a project called Sonifight, which looks as though it might be very helpful for us. I might be able to make X3 voice its own menus, or at least have this program help us do so. THis is still a ways away, as I"m still getting familiar with how it works, but it's quite promising.