I was looking at donating Bitcoin to the Free Software Foundation and took a look at their PGP signed message here. However, I have three questions regarding the signed message.
1.) While I was reading the message I saw the text below saying to "verify each address, line and signature." How would one go about verifying these?
The following lines containing each address, owner and date are signed by their respective private keys, suggesting that they are valid, controlled by the FSF and this GPG message signer attests that they are to be used for donations. Please use each client software to verify each address, line and signature. 1PC9aZC4hNX2rmmrt7uHTfYAS3hRbph4UN email@example.com 20130802 pG+OoJaENOBvY2rPqB8s8Vssmb+/aw7SgE/2TLtZVmtVO8be5eYBXt4CTqbQo0YzcN97eGz8Fv8GpaqhdV2NgrfI=
2.) If I were to send some bitcoins to the Free Software Foundation and would like for them to know they came from me, I would have to provide some sort of proof. The following excerpt from the message says how to do this; however, I still do not quite understand how to execute this:
Request Receipt of Donation =========================== If you would like for us to know who the contribution came from, and to be able to acknowledge receipt of your donation, then please email <firstname.lastname@example.org> with your name, mailing address, and contribution info including your txid. Signing your txid with one of the sending keys as visible in the block explorer is optional but it can serve as proof that your contribution really came from you. We are a 501(c)(3) charity.
I know that the txid is a unique id of the transaction and hash, but how what would a "sending key" be? How would I then proceed to sign the txid with the sending key?
3.) I used Gpg4win to verify the whole message by importing John Sullivan's GPG public key from here and clicking the "Verify" button. But how does this work and what purpose does the PGP signature at the bottom of the message serve?