You may be excellent in listening but not in writing. If you read your application guide, they strongly recommend that you take IELTS unless English is your first language. I would recommend that you do that, also because then you will know exactly how many points you have for English before you apply so you will know your total points.
If you choose to provide a letter, what may happen is that the immigration officer may award you any number of points that they like, could even be zero or they may ask you to take IELTS. You will then have a time limit to take the IELTS and send the results and what can happen is that you do not get as many points as you had hoped for and end up with less than 67 points total and lose your application.
If you take IELTS before you apply, at least you will see right away what your total is and have the chance to improve your English and take IELTS again before you apply.