do you need to put a stamp on priority mail flat rate ?
No, just print out a shipping label for it or head to your local post office and they'll label everything for you.
The shipping label Alex refers to is the one you create at usps.com. On the main page, click "Calculate a Price" and go through all the steps. For most package categories, you can purchase and print a shipping label that has a QR code as a "stamp" on it, indicating you paid for shipping that item.
Just don't reuse the QR codes. It won't work. Once a QR-code "stamp" is "canceled" (used), the postal database doesn't allow a reuse of the same unique code.
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