Why can't I ship Fedex Smart Post to a PO Box on Ebay Labels?

PO Boxes
Actually, you can. Fedex says "By utilizing the U.S. Postal Service® (USPS) for final delivery, FedEx SmartPost reaches every U.S. address, including P.O. boxes and military APO, FPO and DPO destinations. You can even use FedEx SmartPost to ship to Alaska, Hawaii and all U.S. territories."
The problem is that some shipping software was written back in the days before Smart Post and UPS SurePost existed and disallows any commercial shipping to a PO Box. Because of this issue, USPS created something called "competitive street addresses" which are essentially the street address of the post office where the PO box is located. Using that address, you can ship via Smart Post and SurePost to a PO Box using any shipping software, including Ebay labels.
Here's an example:
Original Address Street Address
John Doe John Doe
PO Box 20015 39 W 31ST ST UNIT 20015
New York, NY 10001 New York, NY 10001

We've created a spreadsheet for looking up the correct competitive street address for a given PO box address. Essentially, you look up the zip code in the list and use the corresponding address, adding "Unit" plus the PO box number.

Sometimes there is more than one post office for a given zip code, so you'd also match the PO box against the list of box number ranges for each address.

It might sound a little more complicated than it is, but the spreadsheet includes instructions with pictures which should make it clear.

The spreadsheet is on Google docs (no login needed):
Smart Post PO Box Address Lookup

Note that some zip codes are absent from the spreadsheet (and from the USPS source for this information). At this time, we are not sure why some zip codes are missing. However, as an alternate method, you can look up the post office for a particular zip code using the find locations tool on USPS.com

PO Box Locator

If more than one post office is listed with the same zip code, you may need to call to find out if the post office box is located there.

It may help to limit the search to post offices and not approved postal providers, but for some zip codes there is only an approved postal provider that actually houses the PO boxes (e.g. zip codes for universities which have their own mail handling facilities).