AP do get paid for delivering mail from China, though it works out to be a small percentage of actual costs AFAIK. Basically, the bigger difference there is in the volume of mail being sent / delievered between two countries, the more is paid to the country which is receiving the higher volume (and I'm guessing the mail sent here from China is significantly more than the mail that's sent from here to there).
They put forward an idea like this a while ago - basically, they were saying mail deliveries 3 days a week, or pay something like $60 a year for 5-day deliveries. I'd be interested to know if they'd stop putting mail / collection cards in my PO box on some days if I did't pay the extra $60 under that plan. 
AP are always claiming they lose $x amount by delivering international mail, but those figures have never been given any context or backed up by anything. I actually thought it was interesting a year or so ago when they increased the prices of domestic services - people went crazy, even more so than the reaction to eBay's new fees, there was people contacting MPs, a petition (which had over 10k signatures in the end) all protesting against APs greed... Then Ahmed comes out in an interview and says "It's not our fault, it's your fault for ordering overseas all the time", and suddenly everyone was trying to see what they could do to change the agreement and get AP more money...
