       Shippex.env #=> :dev
   """
   @spec env() :: atom
-  def env do
+  def env() do
     case Application.get_env(:shippex, :env, :dev) do
       e when e in [:dev, :prod] -> e
       _ -> raise InvalidConfigError, "Shippex env must be either :dev or :prod"

  Fetches the label for `shipment` for a specific `Service`. The `service`
  module contains the `Carrier` and selected delivery speed.

      Shippex.fetch_label(shipment, service)
  """
  @spec fetch_label(Shipment.t, Service.t) :: {atom, Label.t}
  def fetch_label(%Shipment{} = shipment, %Service{carrier: carrier} = service) do
    Carrier.module(carrier).fetch_label(shipment, service)
  end

  @doc """

      Shippex.carriers #=> [:ups]
  """
  @spec carriers() :: [Carrier.t]
  def carriers() do
    cfg = Shippex.config()

    ups   = if Keyword.get(cfg, :ups),    do: :ups
