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If you're buying the postage for it from an APC, then the date won't matter. It's the same as if you were affixing postage stamps, with the exception that you can drop it into a P.O. box even if it's over 13 oz. because you used a credit card to pay for it so the purchase is trackable. If it's a metered postage strip from the P.O. counter, then the date does matter.