How to add custom routes to resource route

In Rails >= 4, you can accomplish that with:

match 'gallery_:id' => 'gallery#show', :via => [:get], :as => 'gallery_show'

Add this in your routes:

resources :invoices do
  post :send, on: :member
end

Or

resources :invoices do
  member do
    post :send
  end
end

Then in your views:

<%= button_to "Send Invoice", send_invoice_path(@invoice) %>

Or

<%= link_to "Send Invoice", send_invoice_path(@invoice), method: :post %>

Of course, you are not tied to the POST method

resources :invoices do
  resources :items, only: [:create, :destroy, :update]
  get 'send', on: :member
end

<%= link_to 'Send', send_invoice_path(@invoice) %>

It will go to the send action of your invoices_controller.

match '/invoices/:id/send' => 'invoices#send_invoice', :as => :some_name

To add link

<%= button_to "Send Invoice", some_name_path(@invoice) %>