E-file I131.

Go to uscis website, find the form I131 and click E-file. You need to file out all the required information and pay the filing fee by a credit card. After clicking submit, a PDF file will be generated. In this file, you get a case number and you are instructed to send supporting document and provided the address to send. At last, you use the PDF file as the cover letter to send all required supporting document to USCIS. The USICS receive date of your application is the time you click "submit" online.
I applied for re-entry permit (Form I131) by efiling last November and got it approved in March. AP should be the similar process.