When purchasing an AR-15 through a retailer, you must complete a few legal documents, including a federal background check. Contrary to myth, you cannot purchase an AR and ship it to your house. You need to send it to a dealer with an FFL (Federal Firearms License), so that they can complete that paperwork and transfer according to US law. If you plan on building a rifle yourself, you will still need to complete a transfer for the lower receiver. All other parts can ship directly to your home.