Wiktionary
n. A machine used to print prepaid labels onto pieces of mail, and to record total postage charges.
WordNet
n. meter for bulk mailings that imprints correct prepaid postage on pieces of mail and records the total charge
Wikipedia
A postage meter or franking machine is a mechanical device used to create and apply physical evidence of postage (or franking) to mailed matter. Postage meters are regulated by a country's postal authority. A postage meter imprints an amount of postage, functioning as a postage stamp, a cancellation and a dated postmark all in one. The meter stamp serves as proof of payment and eliminates the need for adhesive stamps.
Usage examples of "postage meter".
If you have a friend working in a large corporation, you can run your organization's mail through their postage meter.
All a three-digit prefix indicates is which city and that the mail wasn't processed through a home or business postage meter machine or at some rural station.
Too bad the postage meter lead fizzled out - the meter was in such a public place any thief could use it.