Looking at the postage costs for gamefly would confirm if the USPS is full of BS or not. If you are a big business and your flat rate items can not be machined the USPS will let you know. If you do not fix it they will refuse to send it. Assuming there are enough subscribers that they send out in bulk mailing. If a bulk mailing does not match what the postage price was the post office will adjust and charge you for it (or just refuse to take it).
The breakages are more likely happening when coming back to Gamefly where random mail is much more likely to have bad crap happen to it.
The idea that blockbuster or netflix get preferencial treatment is insane. The rules for mailing are actually quite strict, and if you do not meet them properly they post office will deny mailing or charge you more because they have to do more to it. They would like to run as efficient as possible.