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Priority Mail Flat Rate was really designed as a consumer product, and is not ideal for most business shippers. View comparisons of Medium and Large Flat Rate Boxes vs. It can be up to 56% more expensive than using your own plain or custom packaging.You can’t use your own packaging, which can impact your customer's experience.
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This simplifies shipping a lot, but that simplicity comes with downsides: The idea behind Priority Mail Flat Rate is that if you can fit something into a Flat Rate-branded box, there’s just one price to pay, no matter how much it weighs or where it’s going.
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Flat Rate pricing makes shipping costs easier to understand, but it's usually more expensive
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