How to Avoid a Huge Line at the Post Office

So my little girls are usually wonderful little cherubs when we go out in public. Well, "usually" may be a little stretch of the truth, but you get the point. They don't throw huge tantrums at the store (I am knocking very loudly on wood right now) or throw their food across the restaurant, or pull other kid's hair at playgroup (I think I just broke my desk I am currently knocking so hard on wood.). Then one day , that all changed.
We had to go to the Post Office and I decided not to take the stroller in because it was going to be quick trip, and when I drove by I could see thast the line was very short. Well, anyone who has young children knows that you need to add about ten minutes to your trip for each time you need to take you children in OR out of the carseat. So, it was no surprise by the time I opened the door to the Post Office, there was a huge line. I should have just turned around and went on my merry way with my lovely children. But, given the history I just spoke of, them seeming pretty happy, and me having a deadline to make with the package I had, I decided to go for it. As soon as I got in line my children turned into gremlins! Fiona wouldn't let me hold her, Zoe ran around the corner to the PO boxes, Fiona followed her, and no one would come back when I called them. After giving up my place in line, I grabbed them to get back in line. They started acting like I was killing them, so I let them go in embarrassment. I repeated this about three times and then I just gave up. They proceeded to run laps around the post office, screaming and laughing at the top of their lungs, hanging from every countertop ledge they could reach and pulling down all of the packaging products. They WOULD NOT STOP. People were staring. People were laughing. Most people were rolling their eyes and giving me the "if that was my kid..." look. My children were going postal. I wanted to cry. There were still about 4 people in front of me. And then it happened. I must have looked so pathetic and so beat down that ALL FOUR OF THEM actually let me go ahead of them in line! I didn't know what I felt more - thankfulness or pure shame.
Luckily for my sanity, my children have yet to be that obnoxious in public since, but I sure as hell have not brought them to the Post Office again!