I agree with the suggestions just to try your best to have a good time and express your love for them. I imagine it will be tough, but as much as possible, focus on being in the moment with them - even if you get distracted sometimes. You have peace of mind in knowing what you are going to do, after all.
If you can't post a scheduled letter delivery, you could set up a scheduled email (I know it's very impersonal) or leave the letters themselves sealed in envelopes, although others might want to chime in as I've heard sometimes these get taken for investigation? If they're far away enough that you need to take a trip, you could just post them normally.