I am not sure if this wedding is already over, but there are a lot of great things you can do to incorporate the idea of love letters and correspondence without getting too them-y as you stated.
1. for favors provide stationary… you can get really nice items at a local mom and pop shop, so they are unique yet not too over priced.
2. table numbers: use envelopes with the table number popping out of the top or on the envelope (and for added fun, inside the envelope have a love letter… a copy of theirs or one from a famous writer… -love letters of great men could be a great resource for this)
3. Talk to your vendors..the florist, coordinator and such should be able to give you lots of other great ideas.