The easiest way to do that is leave a check for the amount in an envelope, made out to USPS, in the mailbox on the next regular mail delivery day with the flag up. The carrier will bring the stamps back the next day. First class stamps are now $0.60 each (12.00 for a book of 20). All first class postage stamps are "forever" stamps.
Some carriers have books of stamps with them. Or, as stated above, they can bring them next day.
Or you can just order them in the website
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