There are discounts to retail USPS pricing if you buy it online through PayPal, eBay, etc... Even if he had gone with the cheaper option (such as a flat rate priority mail), there is a discount. But the discount would be less net money on a lower cost option like the $6 priority mail flat rate. So I'm guessing he wanted to capture the biggest "spread" using the discount, hence picking the higher retail option, in order to pocket $2-3 per transaction on the actual shipping vs. retail amount.
That said, I generally don't consider doing something like that a horrible thing. eBay sellers do it all the time. After all, there ARE real "handling" costs that consumers are very averse to seeing itemized, and you can recover a little of that by pocketing the discount. Of course in this context, and considering the product, I totally understand the complaints...