The best type of brownies to ship are those that are chewy or fudgy. The cake-like ones are more fragile and could arrive at a less appetizing state. Freezing brownies before shipping is also not advisable as they get soggy and melt when exposed to low temperatures.
To package brownies for shipping, wrap each brownie in plastic wrap. Get an air-tight food plastic container or just re-use a metal cookie tin and line it with aluminum foil. Arrange the brownies the container, making sure that they fit snugly and does not move or shift around.
Get a shipping box that can accommodate the brownies container with just a few inches of extra space on all sides. Layer some packing peanuts at the bottom of the box, place the container on top and fill the remaining spaces with more packaging materials. In lieu of packing peanuts, a crumpled newspaper can also do the job of keeping the container in place. Enclose a sheet of the destination and return addresses and phone numbers inside the box just in case the ones outside were accidentally removed. Close the box shut and seal with packaging tape.
Attach address and labels on the box and take the package to the nearest post office. Choose overnight delivery or the fastest option available.