how to select particular product from bunch of products
You could use the Rules module to react on a node save, check if the field has only one value and alter the values of the node as needed.

Or you can write your own module and use the form_set_value() function to change the submitted values in a (custom) validation function. Your validation function can be attached to the node edit form by implementing hook_form_FORM_ID_alter() and adding the function to the $form['#validate'] array.