Peace of mind. Imagine you receive your cover letter back from your usual editor. In between the words you wrote are red tracked changes and comments. Now hit the 'Accept All Changes' button. As you do, you may have some doubts: 'Were the changes grammatically correct?' A native speaker could answer that question; an expert in grammar could too. Now consider, 'Did the editor find all the errors, or will some errors remain in your edited text?'.
We edit your cover letter twice. If editor 1 makes the same change as editor 2, it is an experts' consensus on what is grammatically correct. We motivate editors to find all the errors by paying an extra share of your budget for every consensus change found.
Feedback. You are welcome to leave some feedback.