I understand sellers want/need protection too, but there should be a better way.
I do think buyers should have the right to have sellers ship in the method they wish.

Maybe a good compromise would be an automated system that the buyer could use (for those sellers that insist on Express) that basically has them waive any rights as far as the buyer protection scheme.
Only of course as far as delivery protection, if the item is faulty or the wrong one the right to claim on that should be left intact.

Of course in an ideal world Paypal would accept the standard Priority Mail confirmation every time, but such is bureaucracy ~shrug~