Hi Slackwater:
I had that specific question for MM a couple of weeks ago when I was ordering (its a process for me... I start out small and keep adding more varieties! ).
I was told they have several size boxes. they go from: ones that hold 25, to 35, to 50, to 100. Yours are close enough to the 35 count box that they may not add any extra roo's, but it is possible they will.
I kept adding to my order till I ended up with 42. The customer service rep from MM asked if it was ok to add packing peanuts (in her words they would be all Roo's) to fill the box. I said I would rather not have any more roosters so she marked on my order to not add packing peanuts. Of course If felt quilty and didn't want my chicks to get to cold so I called and ordered more chicks so that I could have an even 50. (I actually got the chicks last night: I ended up with 53: two extra females: a white leghorn and a white rock female, plus the free .... rare chick).