Metal Gear Solid 5: The Phantom Pain's huge open-world environments can take a long time to traverse. Fortunately, the developers included a way to fast travel. Even players who have spent dozens of hours in the game don't know about it, though.
The biggest enemy camps have delivery points, which look like metal orange squares with railings on two sides. You'll find a sign right next to the delivery point with an invoice attached to it. If you grab the invoice, you'll then unlock that point for fast travel.
If you have multiple points unlocked, you can use fast travel by using one of your cardboard boxes and then hiding on the delivery point. A truck will then pick you up and deliver you to a base of your choice. The big catch here, though, is that you have to be stealthy. If the enemy is on alert, you can't use the fast travel.
There are nine fast travel points in Afghanistan and twelve in Africa. Again, they're located at key enemy bases. Don't bother looking in tiny guard posts with a couple guards. If you're having trouble tracking down the delivery points' exact locations, watch these videos from PowerPyx:
Mother Base also has delivery points of its own so you won't need to use the helicopter or jeeps to get from one platform to the next. Here's a video from Random Chievo's showing where you can find the delivery points throughout Diamond Dogs' headquarters:
If you collect all the invoices in either Afghanistan or Africa, you'll unlock development of the C.BOX (WR), a water-resistant box for Snake/Big Boss that won't be damaged by rain. Should you manage to collect all the invoices in both countries, you'll get C. Box (SMK). This box releases smoke when destroyed to cover your escape from enemies.
I'm surprised that Metal Gear Solid 5 doesn't announce this feature more clearly to the player. It can save you a huge amount of travel time throughout the game. However, the Metal Gear Solid franchise has a proud tradition of Easter eggs so I suppose I shouldn't be so shocked.