I had the same problem so I couldn't sign up and get rewarded, but signed up with my post office box for an advertisement through the mail and they are delivered to my post office box by USPS w/o fail.
I talked to my post office since then as I ran into this with something else, and they gave me the physical address of the post office and then I just add my box number to the end of it. It has worked so far....
I thought I used my PO Box for the Disney Movie club. It always gets delivered to my PO box. Mail is not delivered to houses in town where we live.
Don't you have a physical address you can use? Our town is so small, that 1/2 the time if you don't even have a street name, it will get to the person. Just call us Andy Griffith world...