There's some pretty simple rules of thumb when it comes to knowing what will be the cheapest way to ship:

  • If your package is 15.9 ounces or less, then using your own packaging with the First Class Package service will always be cheapest.

  • If your package is over 1 pound but it fits into any Flat Rate Envelope or the Small Flat Rate Box, those are the cheapest ways to go.

  • If your package is over 1 pound and doesn't fit into those, then it's cheapest to use Parcel Select Ground, as long as you're okay with 2-8 day delivery speed.

  • If you need a faster service, use as small of a box as possible to qualify for Priority Mail Cubic.

  • Only if your package weighs more than 20 pounds or you're shipping to the farthest zones and it weights more than 2-3 pounds would Medium or Large Flat Rate Boxes ever be a good deal. Regional Rate Boxes are never a good deal.

One benefit of Priority Mail Cubic is you can use your own packaging, but USPS also offers free boxes you can use with Priority Mail Cubic: 

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