Great, thanks for that link. The packaging requirements on that page do not exclude rigid mailers, as the bulleted list cited elsewhere (apparently derived from the shipping label purchase page) does. And the linked page indicates that sellers can mail "no more than 15 raw cards per package per envelope (or up to 2 trading cards with top loaders)" which further implies that a rigid envelope is acceptable if it falls within the other limits (e.g., thickness and other dimensions, weight, etc.).
