Ruby on Rails - POST requests too many times

I have a problem with my shopping cart app: when I press the add-to-cart button, the POST request is submitted multiple times. For some reason, it is usually rising with a multiple of three. For example, the first time the POST request is sent 4 times, after refreshing 7 times, 11 times, ...

I honestly have no idea where to look for the error, but here is some code:

ApplicationController:

before_filter :current_delivery
def current_delivery
  if session[:delivery_id]
    @delivery = Delivery.find(session[:delivery_id])
  else
    @delivery = Delivery.create
    session[:delivery_id] = @delivery.id
  end      
 end

OrderItemsController:

  def create
    @delivery.order_items.new(order_item_params)
    @delivery.save
  end

ProductsController:

def index
  @products = Product.all
  @order_item = OrderItem.new
end

Products index:

<% @products.each do |product| %>
  <%= render "product_row", product: product %>
<% end %>

=> Partial product_row:

  <%= form_for @order_item, remote: true do |f| %>
      <%= f.number_field :quantity, value: 1, class: "form-control", min: 1 %>
        <%= f.hidden_field :product_id, value: product.id %>
        <%= f.submit "Add to Cart", class: "btn btn-primary" %>
  <% end %>