A few years ago, I used to put my "model airplane" in a bike box.
$25 each way ("bikes" were $75-ish).
Obviously, the rules for oversize has changed in the ensuing years, but the basis might be consistant…"Bike" falls under a header with a price, "model airplane" is not listed [HTML_REMOVED] they must be resorting to "misc" oversize.
Of course, the real problem comes when things go missing [HTML_REMOVED] your model airplane suddenly gets declared as a $$$$ bike.