Imagine an NPC (~7th level spell caster with limited resources) gives a letter to a low-level party (levels 1-2) asking them to carry it to another NPC (~7th level something with a lot of resources). The contents of the letter (as the party suspects) make it very tempting to open and read it. What can the NPCs do to discourage the party from doing that (aside from asking them nicely not to)? I don't want to put traps inside the letter, the goal is to make it risky to open and read it. Making it very likely that the recipient recognizes (with or without magic) that the letter was open is probably enough. The party should have an option to do that if they accept the risk. They should also have a chance to succeed.
I assume that a typical wax sigil could be repaired with Mending, the same goes for similar mundane protections (I suspect my players to argue that and I'm inclined to agree, unless there are RAW to prove otherwise). However, perhaps a skill check for carefully breaking the sigil is in order to make it harder? (Disable Device?) Anything else?