I found a way to circumvent the bug with multiple postings, granted you have enough bagspace. You have to split up the stacks prior to posting them. This way the addon won't have to split the stacks, avoiding the problem of getting bigger stacks on the AH than intended.
I'll look at the LUA that cnzjs posted later tonight, but I'm quite new to LUA as well. Maybe someone more experienced could take a look at this instead.