discovering an authority endpoint

Looking at the Web Payments 1.0 spec, I see:

---------------------------------- Step #3 ----------------------------------

*** Agent HTTPS POSTs the Transaction object to the authority

POST transactions HTTP/1.1
Host: bluebank.com
Content-Length: 634
Content-Type: application/ld+json

{
   "type": "Transaction"
   "transfer":
   {
      "type": "Transfer",
      "amount": "7.50",
      "currency": "USD",
      "source": "https://bluebank.com/people/john/accounts/party-funds",
      "destination": "https://redbank.com/people/jane/accounts/hosting",
      "comment": "Chipping in for the Pizza Party tomorrow."
   },
   "signature":
   {
      "type": "JsonLdSignature",
      "created": "2011-09-23T20:21:34Z",
      "signer": "http://bluebank.com/people/john/keys/5",
      "signatureValue":
"ExMgoDIyOGQzNGVkMzVmmMzYzI43Q3OQzNmOWM32Nj4NTIyZTkZDYMI="
   }
}

Some questions:

1. How do I know where to POST this file to, given that I may know the
authority already?

2. Can I use PUT as well as POST?

3. Is signature mandatory, e.g. if user is already authenticated,
perhaps it's not always needed?