       Shippex.carriers #=> [:ups]
   """
   @spec carriers() :: [Carrier.t]
-  def carriers do
+  def carriers() do
     cfg = Shippex.config()
 
     ups   = if Keyword.get(cfg, :ups),    do: :ups

  end

  @doc false
  def config do
    case Application.get_env(:shippex, :carriers, :not_found) do
      :not_found -> raise InvalidConfigError, "Shippex config not found"


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

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

  @doc """
