If it arrived within the 60 'window' you will be fine ... in that regard.
Yes save the FedEx proof of delivery.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
Your application will not be treated any differently because it was received on the 60th day as opposed to the first day of eligibility. Impressions or intent does not count here; short of an "act of god" the bright line 60 day window is failsafe. Count your blessings, and save all evidence.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter, nor should it be viewed as establishing an attorney client relationship of any kind.
Please make sure you save all the evidence. Also, you should consult an immigration attorney.
This answer is not intended to replace a consultation with an attorney and does not create an attorney-client relationship.