A PO Box would cost more than it's worth to maintain. The yearly fees are, in part, based on the size of the box and what fee group you're placed in.
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[url=http://pe.usps.com/text/dmm300/notice123.htm#2568215]PO Box fee list
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[url=http://pe.usps.com/text/dmm300/notice123.htm#2568215]PO Box fee list
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