resources :line_items do
    post 'decrement', on: :member
end

def decrement
    @cart = current_cart

    @line_item = @cart.line_items.find_by_id(params[:id])
    @line_item.decrement_quantity

    respond_to do |format|
      if @line_item.save
        format.html { redirect_to shop_path, notice: 'Line item was successfully updated.' }
        format.js {@current_item = @line_item}
        format.json { head :ok }
      else
        format.html { render action: "edit" }
        format.js {@current_item = @line_item}
        format.json { render json: @line_item.errors, status: :unprocessable_entity }
      end
    end
  end

<%= button_to 'X', decrement_line_item_path(item)  %>

No route matches [POST] "/carts/25"

No route matches [POST] "/carts/25"

# in config/routes.rb
resources :carts do
  resources :line_items
end

resources :line_items, only: [] do
  post 'decrement', on: :member
end

resources :line_items do
    post 'decrement', on: :member
end

decrement_line_item POST   /line_items/:id/decrement(.:format) line_items#decrement
         line_items GET    /line_items(.:format)               line_items#index
                    POST   /line_items(.:format)               line_items#create
      new_line_item GET    /line_items/new(.:format)           line_items#new
     edit_line_item GET    /line_items/:id/edit(.:format)      line_items#edit
          line_item GET    /line_items/:id(.:format)           line_items#show
                    PUT    /line_items/:id(.:format)           line_items#update
                    DELETE /line_items/:id(.:format)           line_items#destroy

<%= button_to 'X', decrement_line_item_path(item), method: :post  %>