My players are setting up a large nexus of guilds from my empire and one or two they made up themselves across several cities. They are having a hard time sending long messages to and from each other, and hiring a mage is not an option.

How can I, as DM, set up a multi-use system for them to send maps, long messages, money, basically anything small to and from places a few hundred miles apart fairly quickly?

I'm looking for a way to homebrew a device that can send and receive the things listed above from wherever my players are in the world.

Speaking stones

Speaking stones (WGtE, p. 44) allow for repeated long-distance communication. The stones allow for the holder to send a short message to any other speaking stone. [H/T Ryan Thompson]

Galder's speedy courier

The Galder's speedy courier spell (LLoK, p. 57) allows for instantaneous long-distance transport of small items. Adding the ability to cast Courier to the Speaking Stones from above should allow for limited transport of the required items.

Modifying the restrictions of the above to allow for use at the desired frequency and carrying capacity and you should have a (mostly RAW) network of devices that fulfills your needs and assists your players.

Unearthed Arcana Artificer makes something for this

If you're open to Unearthed Arcana options, the Artificer is able to use their ability to create the Many-Handed Pouch. This seems like an excellent option, although you may need to deal with the fact that the Artificer who creates it can't infuse anything else as it would create the end state of the item (see below for full description):

The infused pouches all share one interdimensional space of the same capacity as a single pouch. Thus, reaching into any of the pouches allows access to the same storage space. A pouch operates as long as it is within 100 miles of another one of the pouches; the pouch is otherwise empty and won’t accept any contents. If this infusion ends, the items stored in the shared space move into one of the pouches, determined at random. The rest of the pouches become empty

There are some potential issues regarding this item, so I'd definitely take a look at some of the other questions here about them so you're aware.

Relatively low-level option: Animal Messenger

The 2nd level spell Animal Messenger, when used on a flying creature, can send a long-distance message, and if the animal is friendly and well-trained, it can easily carry a small bit of cargo, like a document or a small pouch of coins. Of course, there is a real-world precedent for this in using homing pigeons to send messages.

Unfortunately, when cast at the base level, the messenger can only travel 50 miles, so in order to get the distances you're looking for, it will probably be necessary to cast it using a 3rd or 4th level spell slot, increasing the range by 100 miles for each additional spell level. Alternatively, it wouldn't be impossible to set up relay outposts every 50 miles along the roads between cities where low-level casters are stationed who can cast the spell at 2nd level as a ritual all day long, continually forwarding many messages per day. This would be a bit like a telegraph network.

The main advantage of this approach is that you don't need to acquire any magic items. However, you will need to acquire the services of either several mid-level casters (around 5th to 7th level to cast 3rd-4th level spells) or multiple low-level casters (3rd level to cast 2nd level spells), and you'll need to train a bunch of homing pigeons, which may or may not be easier than acquiring a pile of magic items depending on the setting.

One important limitation to keep in mind is the speed of messages: 50 miles per 24 hours. This means it will take several days to send a message a few hundred miles. This is probably fine for most day-to-day guild business, especially considering that all other mundane communications are similarly limited. However, it may not be ideal for emergencies.

As an alternative to Animal Messenger, you could instead use paper birds. From fandom:

After writing a small message (of 50 words or less) onto a paper bird,
the sender whispered the name of the intended recipient. This caused
the paper bird to magically fold and seal itself and to take flight,
taking the most direct route toward the recipient at a top speed of
8.1 mph (11 km/h). Upon arrival at the proximity of its intended recipient, the enchantment wore off and the paper bird became a
regular piece of parchment.

Paper birds were usually found in small boxes containing between 4 and
9 units.