While I don't have any insight into the details, one fellow in California assured me he could - just barely, and to his own surprise - pack a frameset with a 63.5 (25") c-t-c seat tube and 60cm top tube within USPS dimensions (i.e. 108" all told, if I'm not mistaken). He initially said he couldn't do it (and that it would therefore cost $250 shipping to Canada instead of $60), then tried it out and made it work. I have no idea how he did it, or whether the precautions he took in packing were really adequate (or whether other dimensional aspects of this particular frame helped), as the deal did not go through. But he assured me it was possible, and with enough confidence to put that on the record on ebay with the USPS rate as the quoted shipping charge.