I've got a noble NPC that will be sending an important letter to the PCs, but I can't decide how that should work. I imagine he'd be paying a courier to bring it straight to them, so size and shape wouldn't really matter. Would he roll it up? Just fold it? Put it in an envelope?
This being a sensitive subject and he a noble, I feel like a wax seal of some sort would make sense. I'm going to make a prop for this, so I want to get it right!
None of the published sources for the Forgotten Realms (for fifth edition or earlier) appears to describe a postal service or mail system. While this does not exclude the existence of one, it is likely that letter delivery is not formalized by a postal system, so "mailed letters" may not really exist in the Forgotten Realms. Instead delivery is carried out by individual messengers. The DMG says on p. 94, that you hire and pay a messenger for your letter, and as such, the form of your letter is entirely up to you.
When adventurers hire a coach to carry them across town or need a letter delivered, the driver or messenger is a hireling (...)
Taking a look at how letters looked like in earth's history during ancient, medieval and renaissance times that match the level of technology in the various parts of the Forgotten Realms, this could range from a clay tablet with cuneiform writing and a seal stampled in, to a scroll as depicted in the beautiful answer by Thomas, but the most common form was a sheet of paper or parchment, folded so the writing was on the inside, and sealed shut with a waxen seal, so it could not be opened and read without the tampering being apparent.
The Equipment list does contain a signet ring, and the Forgery Kit does contain papers and parchments, pens and inks, seals and sealing wax.
This concept of wax-sealed letters can also be seen in Candlekeep Mysteries (p. 14), where a letter of admission is required to enter the Inner Court:
It’s customary for the beneficiary of this consent to receive a signed and sealed letter of admittance. The wax seal must be intact and unbroken when the letter is presented to the Keeper of the Emerald Door.
Envelopes are a relatively modern invention and did not become common before the 19th century. However, there is written evidence that they are sometimes used already in the Forgotten Realms, here form Candlekeep Mysteries, p. 56:
A search of the room yields a pile of crumpled, half-written letters on the fireplace mantelpiece, beseeching other nearby settlements for aid; among them is a completed letter (...). This letter is stuffed in an envelope and sealed shut with a bright red wax seal
Note that a letter of recommendation, such as depicted in Thomas answer, is different in nature from a letter that you would send: you want to open and show it multiple times to multiple people, so a seal on the outside does not make sense here, while a seal on the inside, to confirm its authenticity, does. You can see that the image of that letter indeed does show it as only closed with string. You would not normally use this method for a letter that you wanted to contain confidential information and that you send to someone through an untrusted messenger.
If you like the form of a scroll for the letter, as you state that the letter contains a "sensitive subject", you could therefore use a wax seal instead of a string wrapping when creating your prop.
The 5e Dungeon Master’s Guide contains this image of a “Letter of Recommendation”:
This would be contained in some sort of rigid tube or container for delivery, as we see on p. 228 of the DMG:
Such a letter is usually enclosed in a handsome folio, case, or scroll tube for safe transport, and it usually bears the signature and seal of whoever wrote it.
And for the letter itself, we have an example from Curse of Strahd of a letter sent by Strahd himself:
While Barovia is technically not the Forgotten Realms, Strahd spent some time in the Forgotten Realms before being sequestered to Barovia. We've no reason to believe Strahd would compose letters differently from others of similar import in the Forgotten Realms.
