You can do that just fine. You can order a minimum of five PCBs and choose to have only two assembled. Then you'll get three unpopulated PCBs and two assembled ones. That's a way to get finished PCBs very cheaply.
Be aware that every component that you use that is not in their basic and preferred extended parts library incurs a one-time three-dollar fee. This hurts disproportionately if your entire design is low count and low cost. So if you always go for the latest and greatest ICs as advertised by TI, instead of the Chinese jellybean clone, you'll add up hidden fees quickly. However, when I choose components, I start out on their basic parts page and only go for a non-basic part if it's not there and I really need it.
