So, you've probably seen or heard the post office commercials about flat rate boxes. I believe, though could be wrong, that they come in 4 different sizes: a small that will run around 5 dollars, two different mediums that are nearly 11 dollars and a larger one that I think is something like 13 dollars.
While this is all great, what I really want to tell you about is the flat rate envelope. You've probably seen them at your local post office. They save me A LOT of money especially with my eBay things.
I live in NY and when I sell something like a pair of shoes for women or a thick flannel for guys, I used to either have to keep the shipping at a high level or lose money when someone from California would buy them. It was tough, because my two biggest buying pools are NY and CA. Also they're biggest discrepancy in cost for shipments.
One day, I ordered a new laptop cord that came in a flat rate envelope. It was strange, because they had it in the envelope in a way that completely seemed unnatural, yet it fit. The wheels began turning.
I decided to try some flat rates out and while I certainly can't fit men's shoes or even a size 10 in ladies, I can fit a number of items that used to cost over 9 dollars via Priority in these envelopes for 4.90. It's also great, because it doesn't leave the shipping a surprise. Everyone knows that they will have to pay the amount stated. I don't have to worry about refunding an overcharge to NY or losing a few dollars to CA, which if I were to sell 5 packages a day to that state (has happened), I would have been out 20 dollars. That starts adding up big time.
My advice is to try it out. You will be seriously shocked at what you can get into one of these things. Grab some next time you're there or you can even order them off the USPS website. They're free to take, so you just have to pay the flat rate once you package it up and bring it back/have it picked up.