Altering Flat Rate Packaging: As long as your Priority Mail Flat Rate box or envelope isn’t sealed with the self-adhesive strip it comes with, you can technically alter the packaging to qualify for weight-based Priority Mail rates... but this almost never works out, and USPS will likely charge you for that specific flat rate service after they deliver your package 🙈
Why we don’t recommend altering flat rate packaging
USPS May Charge You for Flat Rate Service Due to its APV System: While USPS should accept altered flat rate packaging and charge only weight-based rates for it, we don't recommend this because we can’t say how individual postal agents or the Automated Package Verification (APV) system will handle these kinds of shipments.
The USPS APV system may still recognize the Priority Mail Flat Rate packaging, even if it appears much different from its original form. If this happens, your Pirate Ship account will be charged for the cost of shipping that specific flat rate service in the first place.
Use Other Free Packaging Options from USPS: Another reason why we don't recommend altering flat rate packaging is that USPS already offers plenty of packaging that you can order online for free.
With all the available options out there, you can easily find a box or envelope that fits your needs… and you won’t have to alter Priority Mail Flat Rate boxes or envelopes at all 😉
Use Non-Branded USPS Boxes if You Plan on Altering Them: Manipulating your packaging in any way is not something we recommend in general, unless you're cutting down non-USPS branded boxes to qualify for a smaller Priority Mail Cubic pricing tier.
Additional Resources