haven't received case number after 20 days for OPT Extension appication, help

Assuming that the OPT extension application was sent to the correct mailbox, you should receive the receipt of filing within 30 days. If your case was rejected for any reason, you should have received the rejected application by now, have corrected the deficiency, and sent it back in. I do not suggest resending another file to U.S.C.I.S. as you may wind up paying twice for the same benefit and possibly repeat a mistake. A USPS receipt unfortunately only says that the application was received and not that it was filed. Technically it is not proof of filing and thus not a document that an employer can accept as proof of authorization to continue employment. That being said, some organizations are more lax than others in asking for immediate proof of continuing work authorization. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.