Separate the box from the postage
A free shipping-supplies listing does not mean the shipment travels free. Check whether you are ordering packaging or buying a prepaid product. Read the product description and the current postage requirements before filling the box.
Match the marked packaging to the service
USPS-produced Flat Rate packaging is associated with a named service. Do not assume a box marked Priority Mail is a plain box for any cheaper service. Use the current USPS product and mailing guidance to confirm how the particular packaging must be used.
Choose by the packed contents
Measure the item with its protective material in place. A smaller box is useful only if the contents fit safely and the package can be properly closed. Do not choose a box from the product name alone: check the current listed dimensions and the difference between inside and outside measurements.
Compare against your own packaging
A flat-rate option is not automatically the lowest price. Compare the current flat-rate quote with a quote for suitable plain packaging using your actual route and packed weight. Include any materials you still need to buy and the service features that matter for your shipment.
Check domestic and international rules separately
Do not carry a domestic assumption into an international shipment. Confirm the selected service’s destination rules, weight limits, allowed contents, and customs requirements. This guide does not publish a fixed price list, because the final amount needs to come from current USPS information.
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Related guides and tools
Continue with the resources that match the next step for your shipment.
Further reading
Updated October 31, 2026. Read the current official guidance for your route before shipping.