Single action per controller ideas

add_item.rb

# app/actions/cart/add_item.rb

class Cart::AddItem
  include Interactor
  include InteractorBroadcast
  include Wisper::Publisher

  delegate :item, :checkout, :variant_id, :quantity, to: :context

  def call
    fail_if_checkout_is_locked
    fail_if_variant_does_not_exist
    fail_if_out_of_stock

    build_item
    set_item_quantity
    set_success_message
    reduce_quantity_to_available_stock

    item.save

    broadcast(:checkout_total_changed, checkout)
  end

  private

  # ...
end

add_item_controller.rb

# app/controllers/cart/add_item_controller.rb

class Cart::AddItemController < ApplicationController

  def create
    action = Cart::AddItem.new(
      checkout: current_checkout,
      quantity: quantity,
      variant_id: variant_id,
    )

    action.subscribe(EnsureValidShippingMethod)

    action.on(:success) do |result|
      flash[:success] = t(
        result.message,
        name: result.item.name,
        quantity: result.item.quantity
      )
      redirect_to cart_path
    end

    action.on(:failure) do |result|
      flash[:error] = t(result.message)
      redirect_back(default: cart_path)
    end

    action.run
  end

  private
  def quantity
    params[:checkout_item][:quantity]
  end

  def variant_id
    params[:checkout_item][:variant_id]
  end
end

routes.rb

get    'cart'              => 'cart#show',                as: :cart
post   'cart/add'          => 'cart/add_item#create',     as: :cart_add
delete 'cart/remove'       => 'cart/remove_item#destroy', as: :cart_remove
put    'cart/apply_coupon' => 'cart/apply_coupon#update', as: :cart_apply_coupon

Note that I've kept the restful method names so you can see at a glance how the action will respond. I also haven't moved 'cart#show' into the namespace yet.