Post office oft-seen phrase, "Do not fold, spindle, or mutilate." We all know what folding and mutilating is, but how do I spindle my letter, and why doesn't the post office want me to do this?

from Dictionary.com
To impale or perforate on a spindle: Do not fold, spindle, or mutilate this card.

A spindle is:
Any of various long thin stationary rods, as:
A spike on which papers may be impaled.

I think they don't want someone to poke a hole or holes in the card or letter. (Not necessarily you, but the delivery people.)

Way back when (1970's!), an office desk would frequently have a spindle on it for storing mail. It had a heavy base and a long thin sharp spindle on which people would impale the mail.