I've found this to be the case too. Annoying isn't it. Do you have a Paypal account? If so you can go straight to their shipping page & ship media mail & first class from there. The link to do this isn't made available if you log in normally, so after you log-in you have to enter the URL directly.
https://www.paypal.com/us/cgi-bin/webscr?cmd=_ship-now
-Dave