Suggestions for sending email to third party merchant when checkout is completed
You can use a rule for that.

Use the action "Send a mail to an arbitrary mail address"

Set the recipients with php, like this (it's more powerful than tokens) :

&lt;?php print _get_custom_emails_list( $node-&gt;field_custom );?&gt;


Add this custom function in your custom module

&lt;?php
// Your custom function
function _get_custom_emails_list( $field_custom ){
  // your query here.....
  return 'test@test.me, test2@test.me'    
}
?&gt;