usps.com profile needs password reset flow

[stuartpb]

Following from #321 and #322 - this profile really ought to have its password reset flow documented, because the fact that (IIRC) it requires security question answers makes it one that you can end up locked out if you use NilPass (and, considering how much of a bear it is to set one of these accounts up, you really don't want to get locked out).