I emailed Subway yesterday and got a response today. They asked for verification of my mailing address.
My post office does not offer home delivery of mail so I have to use a P.O. Box. Many (if not most) contests do not allow P.O. Box entries and the Post Office refuses to deliver mail that does not have the P.O. Box number on it. It is a Catch 22. (I've often wondered how many contests I've actually won but could not receive the prize because of my mailing address.) Any way, the postmaster told me to go ahead and use my street address but to follow it with "# 9" or even "Apt. 9". She says as long as she sees "9" on the envelope, then she is allowed to deliver it.
I am guessing that Subway checked the delivery address before sending the card and saw that there are no street addresses in my Zipcode and that may be why they questioned my address. FedEx and UPS will deliver to my home/street address without any problem. Only the USPS has a problem with it.