Yup, definitely learned my lesson. Media Mail + buying a box is more expensive than Priority Mail + free box, so I'm just going to use Priorty Mail going forward. I save money and buyer gets 2nd day.
Edit: The frustrating part for me is that my post office accepted the package and didn't say anything. Would have been nice to have been told the box type has to match the shipping type so I'd have an opportunity to correct.