LPT: don't use the USPS flat rate boxes
If you go to the post office looking to mail a package, they may offer you a flat rate box. These boxes are also available in other places like office supply stores that do shipping. They will tell you that it is a flat price regardless of what you put in it, or where it is going, and is therefore more convenient.
That is true, but what they don't tell you is that in the majority of cases, it will be more expensive than regular non-flat rate pricing. In most cases, it will be cheaper for you to use your own box and pay by weight and distance. You can reuse any box (e.g., Amazon) or buy a new one.
The only exception is if what you are mailing is particularly small and heavy, AND is going a long distance (like across the country). In that case, flat rate might be cheaper. Based on my experience, flat rate is cheaper about 10% of the time.