Letters can be used in a Mailbox to be sent to other mailboxes or Trade Stations. For a letter to be sent, you need to right click a mailbox with the letter in hand. For a letter to be able to be sent, you need stamps. Stamps come in different qualities. For the letter to be sent, you need a 1n stamp at the least. For it to be sent with 1-64 of a single item, you need 2n at the least, the cost increases by 1n for every extra different item sent.