operation.rb

class Cart::Checkout < Operation::Base
  call do
    Order.create(...)
  end
  
  before do
    cancel! if cart.empty?
  end
  
  around do
    ...
    yield
    ...
  end
  
  after(:success) do
    send_customer_email_notification
    send_admin_email_notification
    ...
  end
end

Cart::Checkout.(cart_instance, customer)