Our feed store (maine) has the same rule. However, I asked if we could pay for 6 and only take home less, and they said, sure. But she recommened taking all 6 anyway, and then giving away any extras we decided we didn't want, which makes sense since there might be some roos in there. She said if we put a notice on the bulletin board we'd be sure to get a taker, especially if it was a 2-month-ish pullet.
That's an interesting thought that ithe 6-chick rule is to discourage impulse buys. And a good idea, I'd say. While we were in the store a mom came in with kids and they were all ooh-ing over the baby ducks. Bucould tell by the quetions they asked that they would NOT be happy with it once it grew a few more weeks. I was holding my breath HOPING they wouldn't buy one until the left (wmpty-handed, thank goodness). It makes me SO SAD when people don't think through things like that.... and then don't take responisibility for thier decisions, and neglect or abandon animals, spouses, kids.......