How can I ship brownies to ensure that they will be fresh upon arrival?

Zip lock bags should be just fine for a 2–4 day delivery status.
I am showing extreme composure by not making “happy brownie” jokes. ;-)

I agree with Coloma but also wrapping them in tin foil tightly should work well too.

Sealed plastic bags in a sealed metal container (They should be on sale after the holidays) should do it.

First, wrap the brownies in aluminum foil tightly. Next, place them in an airtight plastic bag with a zip at the top. Then, place them in a box and surround them with peanuts in the shell. Bag tightly. The peanuts should absorb any moisture, plus having a few peanuts to go along with your brownies.

A double treat!!

Don’t you want my address?

Ziploc has a vacuum sealer set that’s pretty cheap and very easy to use (especially if you will be doing this often). It uses special Ziploc bags and a hand vacuum for the sealer. Since it’s a manual pump, you can control how much air you take out of the bag to be sure you don’t squish the brownies. If you do this, they will arrive fresh without worrying about any other wrapping/containers. This is how I sent cookies and brownies to my husband when he was in Iraq. The packages would take between 10–14 days to get there (usually) and the cookies and brownies were always fresh.

My friend priority mailed me brownies (I think it took 2 days) just in a zip lock bag in a box and they were totally fine. In fact I left them out and didn’t refrigerate or freeze them for a week and a half and even then they were barely stale. You got some time.