-Right. Because you have had the petition letter, the cover letter is not necessary. But it is also fine for you to have a cover letter, which should be very simple, and just tell what petition you are filing and what documents you provide for the NIW I-140 petition. For example:
"I am filing the immigration petition of National Interest Waiver (NIW) for myself. I enclose the following documents for my NIW petition:
1.
2.
3.
......"