that seems to be the case from here: http://www.ups.com/media/OnLineTools.pdf http://www.ups.com/content/us/en/resources/ship/packaging/guidelines/how_to4.html also, googling shows others experiences regarding this and additional info.
I guess whoever wins can decide where they want it mailed, I'll put the address in when I order it and then if it "barks" at me and doesn't like it, I'll just notify the winner to give me a street address. Most have street addresses that I have dealt with on listia anyways, so hopefully won't have to worry about it.
I am reading the comments above and I am pretty sure from experience it is the company that will NOT ship to po boxes, they usually say no po boxes when it is dealing with food products. So even if the FREE Shipping is unconditional there is nothing that can be done when it is unavoidable when a companies policy says they will NOT ship to a po box. Do you really want food sitting in your po box anyways??