Media Mail can be a substantially less expensive shipping choice for heavier items like books when compared with Priority Mail. Media Mail rates start at only $2.66, whereas Priority can start at $6.70 (as of the time of this writing) depending on the distance to one's destination (Zone-based). Media Mail also offers free package tracking (this used to be an add-on cost), so following your valued items in transit is quick and easy.
While this may not be a significant concern for many people, Media Mail does incorporate longer delivery times - often up to 10 days. The bigger downside, however, comes in the form of Media's shipping restrictions and inspection disclaimers. While the USPS does offer a brief explanation of the qualified materials that can ship via Media Mail (e.g. printed books, CDs, DVDs, etc.), it leaves a lot up to interpretation, and different offices and agents might have different standards when it comes to applying and enforcing these rules. Shipping Media carries an implicit agreement that any packages can and may be opened for inspection to ensure that standards are being adhered to, and should an agent find that the contents of a package violate the terms of shipment, they are able to upgrade the shipment method (i.e. require that the package be shipped Priority instead), with additional charges being billable.
We try to be as careful as possible when packing items for shipment, and typically add extra protection (e.g. bubble wrap, cardboard, etc.) whenever possible. We believe that the likelihood of Media Mail packages being opened, inspected, and then resealed could work against these measures, as we would have no way of controlling the degree of care that would be exercised during that process. In addition, some of our art and guide books could contain show-related promotions or other text that the USPS might find in violation of their "no advertising" policy. This creates a situation where we would need to scrutinize each and every book ahead of time in order to ensure that it was free from such materials, possibly disallow certain titles from shipping this way, etc. Should a package's eligibility be called into question during its transit, we most certainly wouldn't want you - our customer - to have to contend with unexpected delays due to items being returned to us, additional shipping charges that could be assessed, etc.
In short, although we'd love to be able to offer a less-expensive option for delivery, Media Mail seems - at this time, at least - to be rather impractical and uniquely prone to complications that we feel would negatively impact your shopping experience with us. We hope you understand - as always, please let us know if you have any questions, comments, etc.