LETTERSHOP SERVICES - Metering

You've no doubt seen a red ink impression in the upper right corner of your mail before.

This is called a meter imprint and is used in lieu of a postage stamp or a pre-printed indicia (permit number).

In addition to being a fast application process, metering has the advantage of allowing the exact postage amount to be affixed to outbound mail.  You're not overpaying the Post Office like can happen with fixed rate postage stamps.