A detailed mailing tips from USCIS can be found at here.
Assemble
Customers who have purchased NIWKit/EB1AKit will encounter the ’Assemble’ button, as depicted in above figure. Upon clicking this button, NIWKit/EB1AKit will automatically compile all necessary documents, including required forms and submission checklists, into a single PDF file ready for filing.
Submission
There are a few things you need to keep an eye on.
Do not use highlighters or correction fluid or tape. The scanners USCIS use will not properly read information that is greyed out, highlighted or corrected using correction fluid or tape.
Do not send the original documents unless specifically requested in the form instructions. If you send original documents with your form when not required or requested, USCIS may destroy them.
All forms and supporting documents submitted must be single-sided, standard 8½ x 11 letter-size pages.
Mark the envelope and the cover letter with the nature and form number of the submission. For example, Original Submission, Application to NIW (Form I-140).
Do not use binders or folders, heavy-duty staples, Sticky tabs.
You can use clippers to secure your documents. As long as the documents stay organized during transportation process, that is ok.
How To Mail your NIW/EB1A Petition?
1. Print the ready-to-file PDF on single-sided, standard letter-size pages as generated by NIWKIT or EB1AKit.
2. Review the forms to make sure the information is correct, and sign at the correct place.
3. Read through the submission checklist on top of the ready-to-file PDF. Check every listed item.
4. Go to UPS/FedEx/USPS, and mail out your NIW petition package.