How to create an Enquiry form with ability to submit a collection of enquiries
Switch to Entityform. This uses the core field APIs. Add an entity reference field on your products and services types. Details from the project page:

The Entityform module enables you to create front-end forms (fieldable entities), which contain fields that you define! These forms use the standard Drupal fields. This means that out of the box, you can use any standard Drupal node field! As of the time this article was written, there were over three hundred &quot;field&quot; modules. And since Entityforms are &quot;Drupal entities&quot;, you will automatically be able to use future &quot;field&quot; modules!
The forms can be used to create custom surveys, petition, and personalized contact forms, and other customized form. (For those of you who have used Webforms, this module brings Webform's functionality into the &quot;standard&quot; Drupal field / entity world.)
As with everything in Drupal, there are Pros and Cons to using Entityform. All though Entityform is a powerful tool, it is not always the best solution. The article When to Use Entityform discusses the advantages and disadvantages that Entityform may have over other methods of creating user &quot;submittable&quot; forms. Do not use Entityform for forms that will contain 150+ form elements

Entity form works well with other modules like Rules so you can get really custom with your behaviours.