Nothing. It's like giving a little bonus to the USPS.
No
No. The Post Office will accept your overpayment and cancel stamp all the postage stamps, making them no longer usable.
It will be posted if the postage due is exceeded. No refunds, no credit vouchers are extended. If you put too much postage on that is just tough.
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if there too little water animal die crops are not growing and if their is too much water flood came
you can die
you get FAT
The post office has the option of returning the letter to sender, if there is insufficient postage.