1) You can respond to those by left clicking the name of the player, sending a message is one of those options (PST means, "Please Send Tell" btw).
2) The Pay 2 Win vendor in Atlas will let you purchase it for free.
3) Same as above.
4) The friends tab on the chat window will let you do those things.
EDIT: Forgot to mention that there is a Pay 2 Win vendor in Mercy and a, "Transact 4 Victory" vendor in Nova.