Can a transfer be done over postal mail? Suppose I'm stuck in the middle of nowhere with no internet access and I want to make a payment to a certain Bitcoin address.

Is it possible to do a transfer via postal mail, and if so what information would I need to send?

Assume the following:


I have both the recipient's Bitcoin address and my (the sender's) own private key(s) memorized or written down on a piece of paper.
The person who receives the mail (my agent) has access to the Bitcoin network and will act on my behalf.
The mail is delivered securely (i.e. no chance of theft)


What information would I need to send in the mail?

Consider the following possibilities:


A)  Trust


I trust my agent to do the transfer to the given address (e.g. the agent is the recipient), but I can't trust my agent with my private key(s)
I trust my agent with all my information (private keys and all)
I do not trust my agent at all

B)  Access to a computer


I have access to a computer, so I can run any program/scripts that may be required for generating any signatures
I don't have access to a computer (and I'm not Rain Man)
A1B1: Assuming you've prepared in advance, there are some nice offline wallet options.
A1B2: This one's tricky. I'm going to assume that you have access to a computer before running off into the woods, or what have you. I'm also going to assume that you know exactly the amount and address in advance.   


Generate the transaction and sign it, but don't broadcast it (can be done with Bitcoin Armory)
Print out the transaction
When you want to make the transaction, mail the transaction to your agent.

A2B?: Easiest one. Just write down your private key and describe the transaction.
A3B?: Well, you're screwed, because the agent could just never make the transaction.