Here's what the USPS said about cards being media mail

EchoCanyon

Thank you for contacting us about mailing baseball cards eligible to be mailed as Media Mail,

Media Mail® - Books, audio-video recordings, and educational materials - Cannot be mixed with clothing or other merchandise
No. Trading cards are not eligible to be sent as Media Mail® and are subject to First-Class Mail® or Parcel Post® rates according to weight.

If I can be of assistance to you in the future, please don't hesitate to contact me.

Thank you for choosing the United States Postal Service®.

bhl2359

Thanks for clearing that up.