Did I fuck up?

Throwaway time.

I am 19 years old, and wanted to buy my first AR as an end of summer gift to myself. I own other guns, but this is my first AR and my first purchase through an FFL.

I bought it online, and called my local FFL to make sure they would handle the transfer. So far so good. Obviously, as I'm checking out, there are boxes I have to check - yes I'm at least 18, no I've never committed a felony, yes I'm in compliance with all other laws etc.

I don't know if it was there before and I didn't notice it, but almost as soon as I checked out I saw a message that said something to the effect of "The lower receiver will be shipped to the designated FFL. The upper receiver will be shipped to your billing address." Now I'm nervous - I know there is a law about buying just a lower receiver under the age of 21, because it could be built into a pistol. But I bought what was advertised as a rifle. I was only asked to certify that I was 18, not 21, and I was not given the option to ship the entire rifle to the FFL (which is what I would've done anyway given the choice).

So my question is, if I show up at this FFL in 2 weeks and try to sign for the lower receiver, are they going to turn me away? Does it make a difference that it was purchased as an entire rifle? Does it make a difference that it is fully assembled, including a rifle stock? If I won't be allowed to walk away with it, what is my best option for cancelling the order?

And most of all, what kind of sketchy business practice is it to sell something as a rifle and then break it down into components that may or may not be legal for your consumers?

EDIT: For the record, I live in a state with no special regulations for "assault weapons". Only federal law should be pertinent here.

EDIT 2: Oh by the way, if you're looking for a sub-$500 Anderson AR, here you go. I only ask that you think of me when you make the purchase