The United States Postal Service has specific rules and guidelines regarding what items can be sent using media mail, which is a discounted shipping rate for certain materials. When it comes to baseball cards, the determination of whether they can be shipped media mail depends on the nature and purpose of the cards themselves.

To begin, it’s important to understand what is considered media mail eligible under USPS regulations. Media mail is a special shipping class meant for certain printed materials including books, printed music, printed educational materials, and other permissible media like sound recordings and video recordings. The key factors that determine media mail eligibility are that the item must be primarily printed text or have educational, intellectual, or cultural value. Photos themselves do not qualify, unless part of an educational printed work.

Now when it comes to sports trading cards like baseball cards, they present an interesting gray area. On one hand, baseball cards feature photos of players and teams that on the surface would not meet media mail standards. Many argue that baseball cards have broader cultural and historical value given they document and preserve the visual history of the sport over time. They could potentially be considered educational as well given they often include stats, bios, and other facts about the players on the cards.

The determining factor seems to be the intent and purpose behind sending the baseball cards. If an individual is solely sending cards as part of a for-profit trade or sales transaction where the primary purpose is merchandise, then media mail would not be allowed. If the sender is donating cards to a library, museum or educational institution where the purpose is to contribute to their educational collections, then media mail eligibility could be reasonable.

Another key factor is that while single cards or small volumes could potentially qualify, large bulk shipments of cards solely for sales/trading would not meet the spirit or purpose of media mail rates. Media mail is meant for sharing printed works of education/culture, not large-scale distribution of merchandise. So even if educational value could be argued, mass shipments would likely be in violation.

There are risks to consider if using media mail incorrectly. The USPS inspects parcels and can charge additional postage fees or even fines if non-qualifying items are sent media mail. They have oversight to ensure proper use to maintain the lower rates. So hobbyists would be wise to not push the limits or take any grey area risks when large volumes or clear sales transactions are involved.

There is no definitive yes or no, as baseball cards straddle the media mail lines. Small donations to educational institutions presenting cultural value could potentially qualify. But large shipments or purely commercial transactions would not be allowed. Hobbyists are best choosing other rate options to avoid any potential issues. The rules are in place to maintain fairness when media mail subsidies are being used. In borderline cases, it’s always best to verify eligibility directly with the USPS rather than face postage due fees retrospectively.

So in conclusion, while conceivably single cards or small donations could meet media mail standards, large scale trading/sales of baseball cards should not utilize media mail rates due to the intent, product nature, and scale involved potentially exceeding the scope of the media mail class. Understanding USPS guidelines fully is key to properly and legally shipping collectibles like cards in the hobby.