Flat rate boxes are a high-profit item for USPS and they are very small. So unless you are sending something that is extremely heavy and dense (like a brick) from one coast to the other, you can ship much cheaper using the #7 or Shoebox (free!) and mailing regular Priority Mail. Using the USPS Click and Ship program also gives you discounted postage. I weigh my own packages and can compare rates using the Click and Ship program. Flat rate is more expensive than regular Priority Mail 99% of the time even on packages traveling long distances.
