Duke Orsino makes "Cesario" memorize one so "he" can recite it to Olivia, Maria forges a love note in order to convince Malvolio that Olivia is in love with him, and Malvolio writes one to the Countess when he's imprisoned in the dark room, although this was more of a hate mail.
Well, it seems to us that all of these letters have one thing in common. Aside from their status as messages of love, they're all associated with deception and dishonesty. First, Olivia totally rejects Orsino's love note as a contrived and insincere declaration of passion, and she's absolutely right. Orsino doesn't mean any of it (not about her anyway). The letter Maria forges to trick Malvolio, of course, is completely deceptive, and Malvolio's letter to Olivia is utterly misinformed. Written words, it seems, are just not to be trusted, especially when they're contrived. Hmm. That's an awfully funny message for a word smith like Shakespeare to send his audience.
The first time we see coinage circulating on the stage is when Viola gives the sea captain a few coins for cheering her up about her (possibly dead) brother in Act 1. For Viola, who also gives money to Feste and poor Antonio as he's carted off to jail, money is a symbol of her generous spirit.
For someone like Sir Andrew Aguecheek, who burns through money like there's no tomorrow, excessive spending and the wasting of ducats is a symbol of his foolishness and excess.
For Antonio, who gives Sebastian his purse, money seems to represent Antonio's willingness to give himself (in friendship, love, etc.) to Sebastian, who thinks nothing of taking it but gives nothing back in return. This is made even more apparent when Antonio is carted off to jail and desperately needs his cash to buy his way out of the jam. Meanwhile, Sebastian is off spending Antonio's money and hooking up with Olivia.
What about Feste? Someone is always giving that guy money in the play. Feste is a professional performer who works hard for his money. (Unlike Sir Toby, who sponges off everyone else.) Feste's really clever when it comes to getting people to empty their pockets for a few jokes. At other times, Feste is paid to run errands, which reduces him to the status of a menial servant. In these moments, the exchange of coins between Feste and the upper-class characters seems to highlight the class difference between "lowly" entertainers and those with power and wealth.
Olivia's the one who seems to be always handing out her jewellery. First, she gives a ring to "Cesario" (which is a secret signal that Olivia's has a crush on "him"). Then, she gives "Cesario" a miniature (a piece of jewellery with Olivia's tiny portrait on it). Duke Orsino also gives "Cesario" a jewel to pass along to Olivia. In these cases, the unwanted trinkets are emblematic of the giver's love, which is ultimately rejected.
Finally, though, when Olivia gives a…