High-Value Secured Shipping
Get an initial investment to purchase a few armored cars; make sure the backs of the cars are windowless, and have Faraday cages as well to help thwart tracking. At various points across their "shipping area", buy plots of land in out-of-the-way places and build vaults that amount to bomb shelters; be sure to build Faraday cages into their construction to continue to thwart tracking devices. Post regular guards outside the vaults, but once the vaults are built, the door should be sealed from the inside (welded, and then ~1-2 feet of concrete poured behind it). Now you have secure storage facilities.
Adding in Ister's excellent suggestion, the cars would have a small "antechamber" with a normal guard -- just enough room for the guard to sit during the ride; beyond that would be a vault. You could also have bulletproof windows in this section, so the guard can be aware of the surroundings in the event of an attack.
As soon as the valuable goods are loaded into the vault and the vault doors are closed, a monk would teleport into the back, and back out to the nearest secure storage facility with the goods; optionally, a second monk would teleport in a dummy package. If the goods are particularly heavy and/or need to traverse exceptional distances, the monks can "bucket brigade" them from secure facility to secure facility (i.e. monk 1 teleports from armored car to facility 1, monk 2 at facility 1 takes it and teleports to facility 2, monk 3 at facility 2 teleports to facility 3, then monk 4 at facility 3 teleports onto the destination armored car.
Now the trick here is, the goods get stopped at the final secure facility for an appropriate amount of time, to simulate actual transit of a vehicle. Only when it's reasonable for a series of armored truck transfers to have made it to the destination city do you actually dispatch the delivery truck to the destination.
On the receiving end, the driver would be driving an utterly empty armored car. As the car pulls up to the destination, once the recipient has been sighted and ID confirmed, a signal is broadcast to the final monk, who teleports into the vault with the goods and back out, just before the vault is opened and the goods are tendered. The vault would be secured electronically, and once the monk is safely out, they'd transmit the unlock signal so the guard in the antechamber could open the vault (so it's not inadvertently opened before/while the monk is popping in and out).
With this method:
- Even if someone attempts to hijack the armored car as it leaves its origin point, since a monk has teleported out with the goods as soon as the car's doors are closed, the hijacker gets nothing because there's nothing in the car. Similarly, if they try to hijack the destination armored car, since the monk and goods aren't in it until the delivery point, they get nothing (in which case you advise the recipient of a delay due to an attack, but assure them that the goods are secure and will be en route shortly)
- If the shipper puts a tracker on the package, the tracker effectively blinks out of existence as it's loaded on the truck (shielding preventing it from transmitting anything) and since the secure facilities are shielded too, the tracker continues to fail. Once it's offloaded from the destination truck (basically as soon as the doors are opened) a tracker can communicate with the relevant satellites/cell towers/etc, but it looks like the package just "appeared" at its destination at the appropriate time.
Offer a 100% door-to-door guarantee, too: "From the moment our truck's doors are closed at the origin to the moment they are opened at the destination, if anything happens to your shipment we will reimburse you ten times the declared value of the shipment." This way if there happens to be an attack at the origin or destination, where the item is actually exposed, it's technically not covered under the guarantee. Beyond that, the shipment isn't anywhere accessible by even the most sophisticated of thieves or crime syndicates (assuming your monks don't get compromised of course) so no one will ever be able to cash in on this guarantee.
Extra layers of obfuscation and security
Have false "secure facilities", with warehouses/garages. Armored trucks go in, armored trucks come out. Anyone trying to follow the truck just sees it go in, then they don't know whether the package of interest has been transferred to another truck, or what's happened. To save money, these could actually be built on top of the actual secure facilities, with the (sealed) entrances contained within, as well as the appropriate ventilation systems to ensure fresh, safe air in the vault. This also allows you to post armed guards around the facility to protect the ventilation systems, while providing a ruse that the guards are just protecting your valuable shipments.
Keep dummy packages on the trucks. Put a duplicate label (from the actual deliverable) onto a dummy package, except with a small identifier that it's the dummy, and actually ship the dummy package via your armored car network. Won't would-be attackers be disappointed when they get a box of rocks! The duplicate labels can perhaps have a component printed in UV sensitive ink, so black lights in the trucks and secure facilities can help differentiate the actual packages and prevent mishaps where a dummy package ends up getting delivered instead of the real deal. Well, both labels would have a UV label, but you'd have a convention where maybe the dummy packages have a number printed in UV that ends in an odd digit, while the real packages have the same number but ending in an even digit.
Have a full armed security escort on the trucks and at the dummy facilities at all times; make it look like you're using this to protect the packages; plus, if someone does attack a truck (or facility) you can put up an honest show of defending it (even if you're not actually defending the secured goods). The security folks can be mundane people -- any private security firm or internal hires of "normal" people. They'll never be inside the backs of the trucks or the actual (or dummy) secured facilities, so they'll never see what's going on. In the unfortunate event of an attack, you're not risking the loss of a valuable monk (reducing the organization's teleport capabilities), but just the usual risks associated with armed/armored transport (you have insurance for your guards right?)
For maximum security, since ventilation is the only potential avenue of risk here, have massive pressurized air tanks buried adjacent to the vault (and sealed in concrete); if the air intake systems detect any contaminants (biological or chemical agents) they seal off the outside airflow and can keep a nominal supply of fresh air flowing into the vault for hours or even days via the tanks, and scrubbers can help recycle air within the vault (removing excess CO2) for even longer operational periods without outside air. Once the air is determined to be clean again, compressors can refill the storage tanks as needed. Periodically cycling air in the tanks is probably a good idea too, because stale air sucks :P
Cheers to Ister who suggested the inner vault and guard in the truck, to cover things up in the event of an attack (i.e. "we saw someone go in the back of the truck but now it's empty!" -- having a guard in there and not having the monk do the teleport until the vault is sealed avoids that).