It's no error, it's profiteering. 

By under cutting external AP pricing they're forcing sellers to either take a loss on calculated post or buy post through eBay. They more sellers they con into using their post, the more profit they make. They would be getting extremely low trade pricing from AP because of the shear volume of post they buy, significantly less than the 'discount' they are giving sellers, therefore increasing their profits. Currently changing all my items to flat rate post, I'll be sticking to external purchasing of labels and my own AP business discount. Ebay makes enough profit off me already.