Oh, don't forget to include form G-1145. It ask the lockbox to email/text you when they accept your application. The email will have your receipt numbers. The emails I received from the lockbox was sent at the same time the official receipt notices where mailed from Chicago.
I think everyone should include G-1145 with any application sent to a lockbox. It just ask for your name, email address, and/or cell phone number.
An accepted application doesn't mean a complete application. If you are missing the payment or a signature the your application will be rejected and sent back to you without getting a receipt. If you are missing required evidence or even a supplemental form (such as I-693 Medical) then your application more then likely will get accepted and a receipt number but you will receive an Request for Evidence for the missing stuff.