If you order from Verizon, they send you a replacement back cover for the phone. Honestly, I can't tell any difference in the thickness at all. It seemed at the time that I put it in, the phone was slightly heavier, but I don't even notice that now.
As long as it's the Moto manufactured extended battery, it'll come with it.
I bought mine through amazon here. $31.56 after shipping and not a Chinese knockoff.