On a $120 purchase, Walmart.com charged $10.35 to ship to an APO address, compared with $2.10 to a stateside address. For most items, Amazon.com charged the same to ship to an APO address as a stateside address. And Target offered shipping on a $120 purchase to an APO address for less than to a stateside address.
EDIT: A commenter on The Consumerist article about this provided a reasonable explanation. The added shipping cost that the Walmart representative cites and the postmaster says doesn’t exist might well be the difference between the bulk rate Walmart gets from UPS and FedEx and use for normal deliveries, and the USPS cost that they have to use to ship to an APO address. That said, Target and Amazon apparently see fit to absorb that cost at least in the majority of cases, it’s unfortunate that Walmart doesn’t, and it’s something that should be taken in to consideration when picking a retailer.