Letters of Trade

LordXenophon on Candlekeep

There is very little detail in the books, as to how these work, but they authorize you to pick up something that has been (or will be) shipped. It is essentially the shipping paperwork. They are not specific to Waterdeep, but are used all over the Heartlands.
In order to make Letters of Trade useful in my campaign, I had to work out a system whereby they would work. This will probably work for you, as well.
When traders arrange for third party shipping to another city, whether by ship, trading coster, or whatever means, they write out two identical documents, which specify where the shipment will be delivered and whatever other terms (such as holding period, pickup date, storage fee, payment due, etc.) were agreed upon. Both are signed by the sender and the shipper. These letters rarely say what is in the parcel. They use code numbers, the name of the sender or recipient, or some other means to identify specific shipments. This way, one barrel of ale, for example, could not be substituted for a different barrel of the same ale - it is guaranteed to be the exact same barrel.
One copy of the letter is sent with the shipment. Because there is no government postal system and there are no mailmen, a drop ship system is used. A caravan may be carrying dozens of third-party parcels. A ship may be carrying thousands. They all end up in one warehouse, where the shipper's copy of the Letter of Trade will be noted in a ledger and filed.
The sender's copy can be sent to the recipient in a variety of ways, but these letters are usually carried by the senders, who will eventually return to their home cities. They may even be travelling on the same ship, but will probably not be. The letters can even be sold or traded, thus the name.
Some letters of trade are not for drop shipments, but are rather authorizations for the bearer to pick up a certain amout of specified goods from the company's warehouse. This is done when a trader in one city wants goods to be delivered in anothe city, where the goods are already available. Only the letter need be sent. These work more like Mercenary Cards, but are claimed in the same way.
To claim the shipment, you go to the specified city, find the specified warehouse and present the Letter of Trade. If the shipment is still there (10% chance for old letters, according to the books), a clerk will match your Letter of Trade with their copy, compare signatures and, if they believe your letter is genuine, they will turn the shipment over to you.
I have further co-opted this system to get players to go where I want. I use index cards for random encounters. In my card file, there is a section with the names and locations of drop-ship warehouses. These are all in cities where I want the players to go. If a Letter of Trade is found, I tell them the address on one of these cards. There aren't very many, so I have no trouble finding one where I want them to go next.
When they claim the shipment, I draw a random card from another section, full of nothing but things I have decided are in these unclaimed shipments. I've seeded it with everything from cursed magic items, to drinks from Aurora's Whole Realms Catalog, to useless documents, to multiple copies of one Dwarven opera, to explanations of how the shipment was lost, to Ardraken's Refreshment Simulacrum. Some are even adventure seeds, such as when the players opened a chest and found the skeletal remains of a murder victim, from a 20-year-old unsolved murder, the investigation of which led to a change in leadership of one of Waterdeep's noble houses.