Prior to 2005 it was consistently less expensive to shop at the local CVS, book store, or radio shack than to order online.
If my business model requires public subsidies for shipping my goods to turn a profit, my business model is garbage since 100% of my profits are literally taxpayer funded. Maybe small quantity mail order batteries from China isn't actually a reasonable thing to have and it maybe actually costs less to buy batteries in giant crates and have them available in local stores.
I might even say that this is obviously the case (for the battery example). Plus you build the local economy rather than subsidizing it's obliteration.
If a middle man was used, like an importer/warehouser, that middle man added their markup on shipping as well, typically 20 to 40%.
Once Amazon or Wal-Mart are the only company left, they will be free to raise the prices to whatever they want.
You need to keep in mind that the postal treaty wasn't JUST for China either, almost every single nation was part of the postal treaty. Withdrawing from this treaty would likely result in increased postage for all international shipments.