<%= simple_form_for(@provider) do |f| %>
  <% Car.all.each do |c| %>
    <div>
      <%= check_box_tag :car_ids, c.id, @store.cars.include?(c), :name => 'store[car_ids][]' %>
      $<%= c.cost %> | <%= c.description %>
    </div>
  <% end %>
  <div class="actions">
    <%= f.submit "New" %>
  </div>
<% end %>

f.input car_ids, ..., :as => :check_box

@provider = Provider.new(params[:provider])

car_ids = params[:provider][:car_ids]
car_ids.each do |cid|
  @provider.cars << Car.find(cid)
end

@provider.save!