Don't know if this will help. but try calling PORS directly, confirm the address for deliveries.
If it's not different to what you've been trying to input on Wal-Mart's website, call the Wal-Mart that would do the delivery directly and ask what address THEY use for PORS.
If it still doesn't work, forget Wal-Mart and try gardengrocer.com Most folks on this website swear by 'em. Maybe a little more expensive, but better selections and they specialize in WDW resort deliveries.