Google Wallet user initiated cancellation failing without calling postback

Google Wallet user initiated cancellation failing without calling postback

I've gotten a subscription product working fine with Google's Wallet for
Digital Goods, but I can't make user's cancelling their orders work
correctly. If I cancel for them via the merchant center, it works just
fine (I get the postback with the orderId as expected). If the user tries
to cancel (and this is the same in the sandbox and in production), they
see the "Are you sure you want to cancel" dialog, and when they say "yes"
it immediately fails. There is simply no delay there.
From everything I've looked at I'm not getting a postback at all in this
case (and I'm handling the postback fine when we cancel from the merchant
center). I've looked through the logs on our web server, as well as
instrumented the postback handler, and it seems to be failing before it
ever gets to my code.
Help?
Erik