GM Supplier price is not below invoice, but it is fairly close without having to do any work. Sometimes there is extra bonus cash for suppliers, but not on Acadia right now. It also limits the dealer to charging no more than $75 on paperwork fees, which I have never seen a dealer charge much more than that, if they ever wanted to charge more than about $100, I would balk at that anyway.
My dealer also said that in order to get Conquest (if it is still going on by the time I see my car), that he would match supplier pricing so I can get that rebate as well.
It is important to remember that whether your order or take off the lot, the dealer is still getting his "holdback" in additon to any net profit between invoice and sales price. There may be additional benefits as well, the profit he makes on your trade, special incentives to the dealer, and any rebates that you fail to get, the dealer will claim for himself. So the whole "everyone gets supplier pricing" last year did not mean that anyone was losing money anyway, but since some folks pay almost sticker, the dealers were leaving some money on the table they normally can probably count on.