I'm currently reading Postal Pleasures: Sex, Scandal, and Victorian Letters by Kate Thomas (2012). All below quotations are from her book.
Before the advent of the modern postal system and the penny black in 1840, sending letters was generally very expensive.
"In 1830 the post was very much the province of the wealthy few [….] and the charges were paid by the recipients of the mail, not the senders." (10). If you were a polite letter-sender, you would often include some coins inside your letter to help repay the cost of your recipient receiving it.
Poorer people developed ways of getting around this entirely, as is recountedin Harriet Martineau's A History of England During the Thirty Years' Peace (1849, vol. 2, p. 425; p. 21 this book):
"Mr. Rowland Hill, when a young man, was walking through the Lake district, when he one day saw the postman deliver a letter to a woman at a cottage door. The woman turned it over and examined it, and then returned it, saying that she could not pay the postage, which was a shilling. Hearing that the letter was from her brother, Mr. Hill paid the postage, in spite of the manifest unwillingness of the woman. As soon as the postman was out of sight, she showed Mr. Hill how his money had been wasted, as far as she was concerned.
"The sheet was blank. There was an agreement between her brother and herself, that as long as all went well with him, he should send a blank sheet in this way once a quarter; and thus she had tidings of him without expense of postage. Most people would have remembered this incident as a curious story to tell: but Mr. Hill's was a mind which wakened up at once to a sense of the significance of the fact. There must be something wrong in a system which drove a brother and sister to cheating, in order to gratify their desire to hear of one another's welfare."
Of course, there were loopholes for the rich:
"Royals, aristocrats, and eventually MPs were able to 'frank' their mail. Such a person could sign his name across the back of the envelope, a gesture of dominion that meant that his letters traveled free of charge in the postbag. Needless to say, franking was a privilege that allowed for extensive and costly abuses" (10).
The footnote which accompanies this paragraph reads, "Two of the most notorious fraudulent frankers were the poets Percy Bysshe Shelley and Samuel Taylor Coleridge. They would approach MPs, pleading extreme poverty, and persuade them to sign the backs of their letters. Shelley also forged the signature of his own father, who was an MP".
However, this all changed for the better when Queen Victoria's penny post revolutionized the mail system. "In celebration of the institution of hte Penny Post, Queen Victoria renounced her own franking privilege. This was a remarkable move: an act, on a certain level, of abdication. By stripping her signature of the power to circulate her correspondence freely in her own dominion, she relinquished her royal privilege and symbolically joined the 'poorer and more numerous classes'. From this point on, cheap communication was to be the right of a nation of citizens, not of a royal household." (14).