From USPS.com:
You can use Delivery Confirmation with:
First-Class Mail® Parcels - Envelopes and small packages weighing 13 ounces or less. Applies only to boxes or envelopes measuring at least 3/4” at thickest point.
Priority Mail® - Cost effective delivery in an average of 2-3 days.
Package Services Parcels - Send small and large packages, envelopes, and tubes. Includes Parcel Post®, Media Mail®, Bound Printed Matter, and Library Mail.
Standard Mail Parcels - Items weighing less than 16 ounces can be sent at discounted presorted and automation rates. Available only for pieces subject to the residual shape surcharge and mailed at the Electronic rate.
Parcel Select® - Send large volumes of packages at a discount.
So the 3/4" thickness requirement applies only to First Class Mail. I have not seen any exception for rigidity of the package, but that doesn't mean drauto is incorrect.
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It's been a little while since we've had a Media Mail thread, so
tony5205 cannot be expected to have seen one.
drauto is correct that items containing "advertising" are not eligible for Media Mail. The problem is that different USPS employees define or recognize "advertising" in many different ways. Ads in non-current comics (back issues) may or may not be seen as "advertising" anywhere in the process from mailing to delivery. If you tell postal clerks that you are shipping comics Media Mail, some will not accept them. Others will not care. Same goes for postal inspectors, who conceivably may be examining your package at any point in the process. If the contents of a Media Mail package are deemed ineligible for Media Mail rates at any time after mailing, they will generally be delivered with postage due from the recipient, usually in the amount of the difference between the rate for Media Mail and Parcel Post, which can be significant. Therefore, if you as a seller ship comics Media Mail, you must be prepared to indemnify your buyers if they face additional expense on their end.
That stated, I get approx. 80% of my comics delivered Media Mail. I mail approx. 90% of comics sales via Media Mail. I have never had a package opened and delivered with postage due either way. I have received a sales brochure from Saab - yes, a giant advertisement with no content other than advertising - via Media Mail.
IMO it is nice to be able to offer Media Mail shipping to your buyers. You are certainly not violating the spirit of the law by shipping comic books via Media Mail. The USPS has become more aware of people shipping non-media items like dishes, clothing, etc. at Media Mail rates, so it is because of that sort of thing that packages are or may be getting opened. Just be wary of overzealous USPS clerks at the time of mailing, state you are shipping "books" (which is not a lie!) and hopefully enjoy a greater eBay sales volume.
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Check out my "ME" page for a link to The Corral - sweet freedom is at hand!