Thanks for replying. For safety, I will use USPS, even though I feel it's odd to have the case denied with Fedex delivery.
package with USPS.Someone did that (sending RFE with Fedex) and got
【在 m****w 的大作中提到】 : You simply CANNOT use fedex. You must use the provided PO Box and send the package with USPS.Someone did that (sending RFE with Fedex) and got : denied.